Dr Raj Kumar
CSIR - Central Scientific Instruments Organisation,
Chandigarh
Overview
 Introduction to basics of laser physics
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Working principle of a Laser
Main components of a Laser
Lasers based on number of energy levels
Lasers modes
Main properties of a Laser
Types of Lasers
 Solid State Lasers
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Ruby Laser: the first laser
Nd: YAG & Nd: Glass Lasers
Tunable Solid State Lasers
Alexandrite Laser
Ti: Sapphire Laser
Colour Center Lasers
Fiber Lasers
 Applications of Solid State Lasers
What is a Laser ?
Light Amplification by Stimulated Emission of Radiation
Spontaneous emission
Stimulated emission
Working principle of a Laser
E2
h
h =E2-E1
E2
h
E1
Absorption
Spontaneous
emission
h
E1
Stimulated
emission
Working principle of a Laser
Let n1 be number of atoms in E1 state and n2 be number of
atoms in E2 then
If n1 > n2
E
• radiation is mostly absorbed
• spontaneous radiation dominates
If n2 >> n1
2
E1
• most atoms occupy level E2, weak absorption
• stimulated emission dominates
• light is amplified
Necessary condition: population inversion
For stimulated emission to dominate, there must be more atoms in
excited states than in ground state. Such a configuration of atoms is
called a population inversion.
Main components of a Laser
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All the lasers comprise of three basic components
• Active medium,
• Excitation
source/pump
• Reflecting mirrors/
resonator
 Lasers differ only in terms of Active medium or Excitation process.
Lasers based on number of energy levels
Three-level laser
• No lasing action in two level system : no population inversion
• Three level system: lasing possible but require high pump energy than
four level system
• Example: Ruby Laser (three level)
Lasers based on number of energy levels
Four-level laser
• Number of thermally excited ions in the lower laser level is small
• Easy to achieve population inversion even by pumping a relatively
small number of ions into the upper laser level
• Lower threshold compared to a three-level system
• Example: Nd: YAG Laser
Lasers modes
Longitudinal mode
frequency separation
• Laser oscillates in a number of transverse and longitudinal modes
• Transverse mode is selected by using mechanical apertures in the cavity
to allow only selected mode and suppress other oscillating modes
• Longitudinal mode is selected by using Fabry-Perot Etalon in the cavity
• TEM00 is preferred for most of the applications
Main properties of a Laser
 Coherence: from phase correlation
 Directionality
 High intensity: results from directionality
Monochromaticity: results in high temporal coherence
Short pulse duration
Types of Lasers
 Several ways to classify lasers
 Mode of operation :
Continuous Wave (CW) or Pulsed
 Active medium:
- Solid lasers
- Gas lasers
- Liquid lasers
- Semiconductor lasers
 Classification may be done on basis of other parameters
 Gain of the laser medium
 Power delivered by laser
 Efficiency or
 Applications
Solid State Laser
• For historical reasons, solid-state lasers are lasers in which
active ions in crystal or glass host materials are optically
pumped to create a population inversion
• Other types of lasers that employ solid-state gain media are
semiconductor lasers and optical fiber lasers and amplifiers.
Since these lasers employ very specialized technologies and
design principles, they are usually treated separately from
conventional bulk solid-state lasers
• Semiconductor or diode lasers are mostly electrically
pumped (though in principle, optical pumping may be
possible with some)
Solid State Laser
Are versatile and provide a large range of average and peak power,
pulse width, pulse repetition rate, and wavelength
The flexibility of solid-state lasers stems from the fact that:
• The size and shape of the active material can be chosen to achieve
a particular performance
• Different active materials can be selected with different gain,
energy storage, and wavelength properties
• Output energy can be increased by adding amplifiers
• A large number of passive and active components are available to
shape the spectral, temporal and spatial profile of the output beam
Solid State Laser: basics
Active centers are fixed /doped (~ 1%) in a dielectric crystal or
glassy material
Electrically non-conducting
 also called Doped-insulator lasers.
• Crystal atoms act as host lattice to active centers
• Crystal usually shaped as rod
• Pumping: Flash lamp or diode laser
• Active centers are from the rare earth, transition metals, or actinides
• Water cooled
Solid State Laser: schematic
Mirrors on both sides of laser rod form a resonant cavity
Solid State Laser: requirements
 Requirements for Host material :
• Should not absorb light at laser wavelength
• Must possess sharp fluorescent lines, strong absorption
bands, and high quantum efficiency
• Crystal should have good thermal conductivity
 Problems with Host material :
o Most of excitation energy ends up as heat rather than light
o Excess heat damages the laser crystal
 Active centres are ions from:
Chromium (Cr), Neodymium (Nd), Titanium (Ti), Cerium (Ce),
Erbium (Er), Holmium (Ho) and Cobalt (Co)
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Chromium is active centre in Ruby and Alexandrite lasers
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Neodymium is active centre in commonly used Nd: YAG laser
Representative Solid State Laser
• Ruby Laser
• Nd:YAG Laser
• Nd:Glass Laser
 Tunable Solid State Lasers
• Alexandrite Laser
• Titanium-Sapphire Laser
• Colour-Centre Laser
 Fiber Lasers
• Erbium in a Glass host
Ruby Laser: the first laser
 First Laser developed in 1960 (TH Maiman)
Ruby laser rod:
 A synthetic pink Ruby crystal (Al2O3 doped with Cr3+ ions)
 Cr3+ ions concentration: 0.05%, Approx 1.61025 ions per cubic
meter.
• Active Centres (Cr3+ ions)
have a set of three energy
• Ruby crystal as cylindrical
rod (4cm length 0.5 cm in
diameter)
• Aluminum & Oxygen
ions are inert
• Helical photographic flash
lamp filled with Xenon.
The Al2O3 (sapphire) host is hard, with high thermal conductivity, and
transition metals can readily be incorporated substitutionally for the Al
Ruby Laser: the first laser
A typical Ruby laser (a) with internal mirrors (b) with external mirrors
Ruby Laser: commercial
 End faces grounded and polished
 Mostly silvered faces (100% & 90 % reflection)
Febry-Perot Resonator
• System is cooled with the help of a
coolant circulating around the ruby rod
 In practical lasers flash lamps of helical
design no longer used
 Most commonly used are linear lamps
Ruby Laser : energy levels
Energy levels of chromium ions is Ruby laser
Ruby Laser : working principle
 A Three level laser system
 E2 - metastable state (3ms)
• Ruby rod pumped with an intense Xenon flash lamp
• Ground state of Cr3+ ions absorb light at pump bands
550nm
400nm
• Non-radiative transitions to E2
• Population Inversion at E2
Radiative transitions from E2 to E1  Red wavelength at 694.3 nm
 Under intense excitation: Pumping > Critical threshold
 A spontaneous fluorescent photon (red) acts as input and trigger
 Stimulated emission; SYSTEM LASES
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Ruby Laser: output
Laser Output: Pulsed with low repetition rate (1 to 2 per sec)
Ruby laser light pulses
• Series of irregular spikes stretching over the duration of pump pulse
• Q-switching concentrates output into a single pulse
Ruby Laser: output
• Stimulated transitions faster than rate at which population
inversion is maintained
• Once stimulated emission commence, the metastable state E2,
depopulate very rapidly
• At the end of each pulse, population at E2 falls below the
threshold value required for sustaining emission of light
• Lasing ceases & Laser becomes inactive
Next pulse will arrive only after P.I. is restored
 High
energy storage capability due to long upper laser level
lifetime
 Pulse energy upto 100J
 Relatively inefficient;  0.1 to 1%
 Variety of applications: Plasma diagnostics; Holography.
Nd: YAG Laser
• Yttrium Aluminum Garnet (YAG) Y3Al5O12 best choice of a host for
neodymium ions (Nd)
• YAG offers low threshold and high gain
• YAG is a very hard, isotropic crystal
• good thermal and mechanical properties
• can be grown and fabricated in rods of high optical quality
• Operation: CW and pulsed mode (high repetition rate)
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Efficiency about 10 times as compared to ruby
Drastic weight reduction
Replaced ruby in military Rangefinders, other applications
Used in the semiconductor industry for resistor trimming, silicon
scribing, and marking
For continuous or very high repetition-rate operation, crystalline
materials provide higher gain and greater thermal conductivity
Nd: YAG Laser
 Active center: Neodymium (Nd) ion- a rare earth metallic ion
 Host: YAG
 Emission at  1.064m
• In Nd:YAG laser, Nd 3+ ions take place of yttrium ions
• Doping conc. ;  0.72% by weight corresponds to 1.41026 atoms/m3
• Rod: 10cm in length, 12mm in diameter
 Nd: YAG rod & a linear flash lamp housed in an elliptical cavity
 In practice, external mirrors (100% , 99% reflectivity) used
 System cooled by water circulation
Nd: YAG Laser
Nd: YAG Laser
lifetime
230 μs
Energy levels of Nd –ions in a crystal
Nd: YAG Laser
 A Four level laser system: Require lower pump energy
• Terminal laser level sufficiently far from ground state
• E3 – metastable level (lifetime 230 μs)
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Two pump bands: 700 nm & 800nm
Pump: intense Xenon flash lamp
Nd3+ ions level E4, decays to upper laser level at E3
Population inversion easily achieved between E3 and E2 levels.
Stimulated to emit 1064 nm laser transition.
From E2 level, Nd3+ ions quickly drop to E1 by transferring energy to crystal
Nd: YAG Laser
 Many other transitions in near IR region; all weaker than 1064 nm
• Only 1318 nm transition produces 20% power as that of 1064 nm
Useful in Fiber Optic Transmission.
Laser Output:
• In the form of pulses of variable repetition high rate
• Overall efficiency  0.1 to 1% range
• Xenon flash lamps : Pulsed output
• Tungsten halide incandescent lamps ; CW output
CW output power of over 1 kW obtainable.
Can be pumped by a diode laser (GaAs) for high efficiency
2nd harmonic generation results in half the wavelength (532 nm)
Nd: Glass Laser
• Glasses are more suitable for high-energy pulsed operation because of
their large size, flexibility in their physical parameters, and the
broadened fluorescent line
• Can deliver much higher energies
• Can be doped at very high concentrations with excellent uniformity
• Practical doping limit is determined by the fact that the fluorescence
lifetime and therefore the efficiency of stimulated emission, decreases
with higher concentrations
• Can be made in a variety of shapes and sizes, from fibers a few
micrometers in diameter to rods 2m long and 7.5 cm in diameter and
disks up to 90 cm in diameter and 5 cm thick
The major disadvantage of glass is a low thermal conductivity
Nd: Glass Laser
 Glass: An excellent host material for Nd
 Attraction for
Glass: well developed technology for making
large size glass (laser) with good optical quality
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While Nd: YAG laser can be operated in CW mode; Nd: glass laser
only operate in pulsed mode because of low thermal conductivity of
glass
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Nd:glass laser very high output energy per unit volume of material
• High energy in short pulses can heat matter to thermonuclear
temperatures, thus generating energy in small controlled explosions
(inertial fusion)
• NOVA lasers developed for Nuclear Fusion by Lawrence Livermore
National Lab. (USA) – employed a large number of Nd: glass amplifiers
to produce 100 kJ of energy in a 2.5 ns pulse.
Nd: Glass Laser
 An inertial confinement
fusion implosion on the
NOVA laser creates
"microsun" conditions of
tremendously high density
and temperature rivaling
even those found at the
core of our Sun.
Tunable Solid State Lasers
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Produce output over a range of tunable wavelengths
 Tunability: existence of a cluster of vibrationally excited terminal
levels near the ground state – Vibronic states
 laser transitions take place between coupled vibrational and electronic
states
 Dye lasers, though tunable, but
suffer from dye degradation and
other limitations
 Solid state tunable lasers have long
self and operational life
Applications: Remote sensing, space, spectroscopy
Tunable Solid State Lasers: Alexandrite Laser
• Alexandrite (BeAl2O4 : Cr3+) is the common name for chromium-doped
chrysoberyl
• Tunability is due to band of vibrational levels which are a result of
strong coupling between Cr3+ ion and the lattice vibrations
• Doping ~ about 0.1% (density~31025 ions /m3); Rod shaped ; 10cm
long, 6mm in diameter
• Pump levels at 380 nm & 630 nm; flash lamp pumped
• Cr3+ levels in Alexandrite form upper and lower vibronic bands
Electronic levels of Cr3+ and vibrational levels of crystal lattice
 Vibronic transitions can occur over a range of energies; excited ion can
drop from upper level to anywhere in lower vibronic band – Gain
Bandwidth
Tunable to any desired wavelength within its emission spectrum
 Can operates in a pulsed or CW mode
 Widely used in cancer therapy, kidney stone removal and pollution
Tunable Solid State Lasers: Alexandrite Laser
Can lase both as a four-level vibronic laser and as a three-level
Absorption bands are very
similar to those of ruby
Energy level diagram for chromium ions in alexandrite
Tunable Solid State Lasers: Alexandrite Laser
• In three level mode laser transition is from 2E state, which is coupled
to 4T2, down to ground state 4A2.
• High threshold, fixed output wavelength (680.4nm at room
temperature) and relatively low efficiency
• In four level mode 4T2 is the absorption state continuum
• Lasing occurs between 4T2 state to excited vibronic states within 4A2
(ground state)
• Laser wavelength depends on vibrationally excited terminal
• Any energy not released as laser photon will be carried off by a
vibrational phonon, leaving the chromium ion at its ground state
(system comes in equilibrium)
Tunable Solid State Lasers: Ti: Sapphire Laser
• Titanium-Sapphire (Ti : Al2O3) laser is widely used tunable
• Broad vibronic fluorescence band allows tunable laser output between
670–1070 nm, with the peak of the gain curve around 800 nm
• Relatively large gain cross section (half of Nd :YAG at the peak of its
tuning range)
• The energy level structure of the Ti3+ ion is unique among transitionmetal laser ions in that there are no d state energy levels above the
upper laser level
 Ti3+ ions replace some of Al3+ ions
 Doping concentration  0.1% by weight
 Operation: Pulsed or CW modes
Tunable Solid State Lasers: Ti: Sapphire Laser
The broad, widely separated absorption and fluorescence bands are
caused by the strong coupling between the ion and host lattice and are
the key to broadly tunable laser operation
Tunable Solid State Lasers: Ti: Sapphire Laser
• Pumping with other lasers like
argon and copper vapor lasers,
frequency doubled Nd :YAG and
Nd : YLF lasers due to short
lifetime of upper laser level (3.8s)
• Flash lamp pumping is inefficient
and requires very high pump flux is
required.
Energy level scheme
Most widely used in laser radar (LIDAR), range finders, remote sensing
and spectroscopy
Colour Center Lasers
 Broadly tunable SSLs – operates in wavelength range of 800-4000nm
 Tuning achieved using different colour-centre crystals in sequence.
 Typical CCL consist of an alkali halide crystal that contains point
defects known as F-centre  Colour Centres
Usually produced when crystal irradiated with X-rays.
 Colour centres remain in crystals for duration ranging few days to
many years.
 Absorb and emit light as the atoms at the defect site change position.
CCLs must be pumped with other laser & maintained at very low temperatures.
 Need for a pump lasers & Cryogenic cooling limits the use of CCLs in
practical application.
Colour Center Lasers: Energy Levels
CCLs must be pumped with other laser & maintained at very low
temperatures
Fiber Lasers
Erbium in a glass host – forms a three level laser with wavelength
centered around 1550nm (range: 1520-1560nm).
1550 nm is important operational window in OFC technology
EDFA is used as an optical amplifier in DWDM technology
Highly useful in undersea and long haul OFC links
Fiber Lasers: Energy Levels
• Needs lasers for pumping to get desired output.
• Output transitions in the range from 1520-1560nm
Optical Parametric Oscillator
• Parametric oscillators based on lithium niobate introduced in 1971
• Discovery of damage-resistant nonlinear crystals with large nonlinear
coefficients in the early 1990s revived interest in OPOs
• OPO can provide tunable range through UV-visible-IR
• OPO works on the principle of non-linear harmonic generation
• In the parametric process, a nonlinear medium (usually a crystal)
converts the high energy photon (the pump wave) into two lower
energy photons (the signal and idler waves)
• Wavelengths of signal and idler beams are determined by the angle that
pump wave-vector makes with crystal axis
• Energy can be transferred efficiently to the parametric waves if all three
waves are traveling at the same velocity (phase matching condition)
• Variation in index of refraction with crystal angle and wavelength
allows "phase matching“ condition to be met only for a single set of
wavelengths for a given crystal angle and pump wavelength
Optical Parametric Oscillator
pump energy = signal energy + idler energy
Signal and Idler beam generated in a non-linear crystal
Optical Parametric Oscillator
Variation of OPO output energy (signal and idler) with wavelength
Diode Laser as optical pumping source
• As diode lasers became less expensive, these are being used as optical
pump in solid-state lasers
• Diode pumping offers significant improvements in overall system
efficiency, reliability, and compactness
• Radiation from laser diodes can be collimated providing great
flexibility of designing solid-state lasers with regard to shape of laser
medium and orientation of pump beam
• In end-pumped lasers, pump beam and resonator axis are collinear
which led to highly efficient lasers with excellent beam quality
• A number of solid-state lasers with outputs up to 20 W are pumped with
diode arrays
• Lasers at multi-hundred watt level are pumped by arc lamps because of
high cost of laser diode arrays
Applications of Solid State Lasers
Solid State Lasers have a wide spectrum of applications
• Materials processing (cutting, drilling, welding, marking, heat
treating, etc.),
• Semiconductor fabrication (wafer cutting, IC trimming),
• Graphic arts (high-end printing and copying),
• Medical and surgical (Welding of detached retinas, correction of
vision defects, surgery, treatment of skin cancer)
• Defence (ranging, anti-missile shield, laser detonators, instruments,
spying and in war time)
• A high energy pulsed YAG laser has even been used in rocket
propulsion experiments
• The largest lasers (with the highest peak power) in the world are solid
state lasers
• Space, remote sensing, spectroscopy, holography
LASER APPLICATION EXAMPLES
Laser for Cutting Fabric in a Clothing Factory
Laser in Material Processing
INDUSTRIAL APPLICATIONS
LASER APPLICATION EXAMPLES
Laser at
War time
LASER APPLICATION EXAMPLES
Laser
fusion
LASER APPLICATION EXAMPLES
HOLOGRAPHY
References / suggested books
 Solid State Laser Engineering , W. Koechner
 Principles of Lasers, O. Svelto
 Lasers and Non-linear Optics, B. B. Laud
 Laser Fundamentals, W. T. Silfvast
Thank you
Why Alexandrite is tunable and Ruby not?
• Equilibrium coordinate for both the 4T2 and 4T1 states, due to their
symmetry, is shifted to a larger value than that of 4A2 and 2E states
• As in other Cr3+-doped hosts, the decay between the 4T2 and 2E states
is via a fast internal conversion (decay-time of less than 1 ps) probably
due to the level-crossing which occurs between the two states.
• These two states can be considered to be in thermal equilibrium at all
times, and, since the energy difference between the bottom vibrational
levels of 4T2 and 2E states in alexandrite is only a few kT, an
appreciable population will be present in vibrational manifold of 4T2
state when 2E state has been populated.
• Invoking the Franck-Condon principle, one sees that the vibronic
transitions from the 4T2 state end in empty vibrational levels of the 4A2
state, thus becoming the preferred laser transition.
•Because there is a very large number of vibrational levels involved,
the resulting emission is in the form of a broad continuous band