Solid State Lasers a

CSIR - Central Scientific Instruments Organisation, Chandigarh

Dr Raj Kumar

Overview Introduction to basics of laser physics

• 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• 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

Absorption Spontaneous

emission

Stimulated

emission

h hh

E1

E2

E1

E2

h =E2-E1

Working principle of a Laser

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

• radiation is mostly absorbed

• spontaneous radiation dominates

• most atoms occupy level E2, weak absorption

• stimulated emission dominates

• light is amplified

If n2 >> n1

Necessary condition: population inversion

E1

E2

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

All the lasers comprise of three basic components

Lasers differ only in terms of Active medium or Excitation process.

• Active medium,

• Excitation

source/pump

• Reflecting mirrors/

resonator

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

• 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

Longitudinal mode

frequency separation

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

Classification may be done on basis of other parameters

Gain of the laser medium

Power delivered by laser

Efficiency or

Applications

Active medium:

- Solid lasers

- Gas lasers

- Liquid lasers

- Semiconductor lasers

Mode of operation :

Continuous Wave (CW) or Pulsed

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)

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

Active centers are fixed /doped (~ 1%) in a dielectric crystal or

glassy material

Electrically non-conducting

also called Doped-insulator lasers.

Solid State Laser: basics

• 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

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

Active centres are ions from:

Chromium (Cr), Neodymium (Nd), Titanium (Ti), Cerium (Ce), Erbium (Er), Holmium (Ho) and Cobalt (Co)

Chromium is active centre in Ruby and Alexandrite lasers

Neodymium is active centre in commonly used Nd: YAG laser

Solid State Laser: requirements

Problems with Host material :

o Most of excitation energy ends up as heat rather than light

o Excess heat damages the laser crystal

• 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

Representative Solid State 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 1.61025 ions per cubic

meter.

Ruby Laser: the first laser

The Al2O3 (sapphire) host is hard, with high thermal conductivity, and

transition metals can readily be incorporated substitutionally for the Al

• Active Centres (Cr3+ ions)

have a set of three energy

• Aluminum & Oxygen

ions are inert

• Ruby crystal as cylindrical

rod (4cm length 0.5 cm in

diameter)

• Helical photographic flash

lamp filled with Xenon.

A typical Ruby laser (a) with internal mirrors (b) with external mirrors

Ruby Laser: the first laser

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

Ruby Laser: commercial

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

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

Ruby Laser : working principle

Laser Output: Pulsed with low repetition rate (1 to 2 per sec)

Ruby Laser: output

Ruby laser light pulses

• Series of irregular spikes stretching over the duration of pump pulse

• Q-switching concentrates output into a single pulse

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.

• 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

Ruby Laser: output

• 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)

• 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

Nd: YAG Laser

For continuous or very high repetition-rate operation, crystalline

materials provide higher gain and greater thermal conductivity

Active center: Neodymium (Nd) ion- a rare earth metallic ion

Host: YAG

Emission at 1.064m

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

• In Nd:YAG laser, Nd 3+ ions take place of yttrium ions

• Doping conc. ; 0.72% by weight corresponds to 1.41026 atoms/m3

• Rod: 10cm in length, 12mm in diameter

Nd: YAG Laser

Nd: YAG Laser

Energy levels of Nd –ions in a crystal

lifetime

230 μs

A Four level laser system: Require lower pump energy

• Terminal laser level sufficiently far from ground state

• E3 – metastable level (lifetime 230 μs)

• 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: YAG 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

Nd: Glass Laser

The major disadvantage of glass is a low thermal conductivity

Glass: An excellent host material for Nd

Attraction for Glass: well developed technology for making

large size glass (laser) with good optical quality

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

Nd:glass laser very high output energy per unit volume of material

Nd: Glass Laser

• 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.

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.

Nd: Glass Laser

http://en.wikipedia.org/wiki/Image:Fusion_target_implosion_on_NOVA_laser.jpg

http://en.wikipedia.org/wiki/NOVA_laser

http://en.wikipedia.org/wiki/Sun

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

Tunable Solid State Lasers

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~31025 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

detection


Can lase both as a four-level vibronic laser and as a three-level

Energy level diagram for chromium ions in alexandrite

Absorption bands are very

similar to those of ruby


• 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 transition-

metal 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


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

Most widely used in laser radar (LIDAR), range finders, remote sensing

and spectroscopy


Energy level scheme

• 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 (3.8s)

• Flash lamp pumping is inefficient

and requires very high pump flux is

required.

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

CCLs must be pumped with other laser & maintained at very low

temperatures

Colour Center Lasers: Energy Levels

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

Highly useful in undersea and long haul OFC links

Fiber Lasers

EDFA is used as an optical amplifier in DWDM technology

Fiber Lasers: Energy Levels

• Needs lasers for pumping to get desired output.

• Output transitions in the range from 1520-1560nm

• 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

Optical Parametric Oscillator

• 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

• Thus as the crystal rotates, different wavelengths of light are produced


Signal and Idler beam generated in a non-linear crystal

pump energy = signal energy + idler energy


Variation of OPO output energy (signal and idler) with wavelength

• 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

Diode Laser as optical pumping source

• 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

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

Applications of Solid State Lasers

INDUSTRIAL APPLICATIONS

Laser for Cutting Fabric in a Clothing Factory

Laser in Material Processing

LASER APPLICATION EXAMPLES

Laser at

War time


Laser

fusion


HOLOGRAPHY


Solid State Laser Engineering , W. Koechner

Principles of Lasers, O. Svelto

Lasers and Non-linear Optics, B. B. Laud

Laser Fundamentals, W. T. Silfvast

References / suggested books

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

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Solid State Lasers a