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Laser Types

Lecture 6

LASER AND ITS APPLICATIONS

421 Phys

Types of lasers

We will somewhat arbitrarily look at lasers based on whether the gain medium is a gas, liquid, or solid.

Gas: - Atomic gas laser (He-Ne laser),

- Ionic gas laser ( Ar Ion laser)

- Molecular gas laser (CO2 lasers, Excimer laser)

Liquid: Dye lasers

Solid: Ruby, Nd:YAG laser, Nd:glass, Ti:sapphire laser

Semiconductor: Diode (semiconductor) lasers

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Atomic gas lasers:

The Helium Neon (He-Ne) Laser

Helium Neon lasers consist of a discharge

tube inserted between highly reflecting

mirrors.

The tube contains a mixture of helium and

neon atoms in the approximate ratio

of He:Ne (5:1).

• By applying a high voltage (a few KV) across the tube, an electrical discharge can

be induced.

• The electrons collide with the atoms and put them in an excited state.

• The light is emitted by the neon atoms, and the purpose of the helium is to assist

the population inversion process.

A typical construction is shown in the Figure.

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The helium atoms can easily de-excited by collisions with neon atoms in the ground

state according to the following scheme:

It would not be easy to get this population inversion without the helium because collisions

between the neon atoms and the electrons in the tube would tend to excite all the levels of

the neon atoms equally. This is why there is more helium than neon in the tube.

Optimum performance in the He–Ne laser is found to occur when the product of

tube diameter and total gas pressure is D × P= 3.6 - 4 torr × mm.

Properties of He-Ne laser beam

He–Ne lasers are low power(a few mW for a laser 10–20 cm long).

The efficiency is quite low, (η ~ 0.02%)

Excellent beam quality

Narrow laser linewidth → high coherence

Many applications, e.g. laser interferometers

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The direct excitation of Neon gas is inefficient, but the direct excitation of

He gas atoms is very efficient.

• An excited state of the He atom has an energy level which is very

similar to the energy of an excited state of the Neon atom

The excited Helium atoms collide with the Neon atoms, and transfer to

them the energy for excitation.

Thus Helium gas does not participate in the lasing process, but

increases the excitation efficiency so that the lasing efficiency

with it increase by a factor of about 200

The role of the Helium gas in He-Ne laser

Ion gas lasers: Helium cadmium

• The population inversion scheme in HeCd is similar to that in He-Ne except that the

active medium is Cd+ ions.

• The laser transitions occur in the blue and the ultraviolet at 442 nm, 354 nm and 325

nm.

• The UV lines are useful for applications that require short wavelength lasers, such as

high precision printing on photosensitive materials.

• Examples include lithography of electronic circuitry and making master copies of

compact disks.

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325 nm

Pu

mp

ing

Ground state

cd+ ion ground state

En

erg

y [e

V]

He+ ion ground state

CadmiumHe

Energy

transfer335.3 nm

441.6

nm

Transitions in He-Cd laser

The excitation to the upper laser level of the Cadmium atoms in the gas is similar to

the excitation process of the Neon gas in a He-Ne laser: Helium atoms are excited by

collisions with accelerated electrons, and than they pass their energies to Cadmium

atoms by collisions.

The transitions in Helium-Cadmium laser are between energy levels of singly

ionized Cadmium atoms, and about twelve lines are available.

These wavelengths are in the shorter wavelength region, violet and Ultra-Violet

(UV). Thus, the main application of the He-Cd laser is in the optics laboratory, for

fabricating holographic gratings.

The practical problem in Helium-Cadmium laser is to maintain homogeneous

distribution of the metal vapor inside the electrical discharge tube.

The ions are attracted to the cold windows at the ends of the cavity. In order to

prevent coating of the windows with Cadmium, cold traps are put before the laser

windows.

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Output wavelengths:

- Blue light 0.4416 [mm]

- Ultra-Violet (UV) light 0.3250 [mm].

Maximum output power:

150 [mW] in the blue line, and 50 [mW] at UV.

Maximum total efficiency:

in the blue line 0.02%, and in the UV 0.01%.

Spectral width:

0.003 [nm] (about 5 [GHz]), and coherence length: about 10 [cm].

Distance between two longitudinal modes:

about 200 [MHz].

Characteristics of He-Cd lasers

Argon ion (Ar+) Lasers

• Argon has 18 electrons with the configuration 1s22s22p63s23p6.

• Argon atoms incorporated into a discharge tube can be ionized by collisions

with the electrons.

• The Ar+ ion has 17 electrons. The excited states of the Ar+ ion are

generated by exciting one of the five 3p electrons to higher levels. The level

scheme is given below.

• The important transitions occur between the 4p and 4s levels of the Ar+ ion.

due to fine structure (spin-orbit coupling) this is actually a doublet.

• The two emission lines are at 488 nm (blue) and 514.5 nm (green).

• Several other visible transitions are also possible, making Ar+ lasers

very good for colorful laser light shows.

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514.5 nm488 nm

Pu

mp

ing

Ground state of Ar

atom

Ground state of Ar ion

Radiative decay

laser transition

En

erg

y [

eV

]

Transition In Ar+ Laser

First: The electrons in the tube collide with argon atoms and ionize them according to

the scheme: Ar (ground state) + lots of energetic electrons

Ar+ (ground state) + (lots + 1) less energetic electrons .

The Ar+ ground state has a long lifetime and some of the Ar+ ions are able to collide with

more electrons before recombining with slow electrons.

this puts them into the excited states according to:

Ar+ (ground state) + high energy electrons Ar+ (excited state) + lower energy

electrons

Since there are six 4p levels as compared to only two 4s levels, the statistics of the

collision process leaves three times as many electrons in the 4p level than in the 4s level.

Hence we have population inversion. Moreover, cascade transitions from higher excited

states also facilitates the population inversion mechanism. The lifetime of the 4p level is

10 ns, which compares to the 1 ns lifetime of the 4s level. Hence we satisfy tupper > tlower

and lasing is possible.

Population inversion is achieved in a two-step process

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The diagram below shows a typical arrangement used in an Ar+ laser.

Argon lasers tend to be much bigger than helium-Neons.

The tube length might be 1–2 m, and the tube might be running at 50 A with a voltage

of 250 V. Hence water-cooling is usually necessary.

Output powers up to several tens of Watts are possible.

The tube is enclosed in a magnet to constrain the Ar+ ions and protect them from

deflections by stray fields.

Metal segmented

structure Solenoid

(magnetic coil)

prism

output

mirror

Anode

Water Jacket

Gas return lineBeryllium

oxide tube

Cathode

The windows at the ends of the

tube are cut at Brewster’s angle

(which satisfies tanθ = n) to

reduce refection losses (there is

no reflected beam for vertically

polarized light at this angle.)

Since there are several laser

transitions with similar

wavelengths, it is necessary to

use a prism to select the emission

line that is to be used.

In addition to laser light shows, argon lasers are used for pumping

tunable lasers such as dye lasers and Ti:sapphire lasers. There are

also some medical applications such as laser surgery, and scientific

applications include fluorescence excitation and Raman

spectroscopy- Microscopy- forensic medicine-ophthalmic surgery

Applications of Ar laser

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Excimer lasers

“Excimer”: excited dimer (E.g., He2) “Exciplex”: excited complex (dissimilar atoms) (E.g., ArF)

Excimers and exciplexes are molecules characterised by a dissociative

ground state, but by a bound potential for an excited electronic state:

Since the lower state is very short-lived, a population inversion can also be achieved relatively easily.

Excimer lasers are pulsed, high power lasers

The name Excimer comes from the combination of the two words:

exited dimer, which means that the molecule is composed of two

atoms, and exists only in an excited state

E.g., ArF (193 nm), KrF (248 nm), XeF (351 nm), KrCl (222 nm), XeCl (308

nm), XeBr (282 nm)

An electric discharge is used to pump the laser.

Note that the Excimer laser can be changed by exchanging the gas mixture (along with the HR and OC).

Although the ground energy level

is short-lived, in some cases the

lower level potential may be very

slightly bound, allowing some

tuneability of the laser.

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4

3

2

1

0

248

nm

En

erg

y [

eV ]

Las

er tr

ansi

tion

s

Kr* + F

Kr + F

r0

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The composition of the gas mixture inside the tube of the Excimer laser is:

Very little halogen (0.1-0.2%).

Little noble gas (Argon, Krypton or Xenon). about 90% Neon or Helium.

The halogen atoms can come from halogen molecules such as: F2, Cl2, Br2, or from

other molecules which contain halogens such as: HCl, NF3.

The advantage of using a compound and not a pure halogen, is the strong

chemical activity of the halogen molecule (especially Fluorine).

Excimer lasers emit in the Ultra-Violet (UV) spectrum.

The radiation is emitted only in short pulses.

The length of each pulse is between pico-seconds to micro-seconds (10-12-10-6s).

The gas pressure inside the laser tube is high: 1-5 [At].

The efficiency of commercial Excimer lasers is up to a few percent.

Operating of the Excimer Laser

Properties of Excimer Lasers

- Active media : excimers, e.g. ArF, KrK, XeCl,…

- Pumping mechanism : electron impact in a gas discharge, ion-ion

recombination, harpooning reactions

- Low efficiency (with respect to the partial pressures of the initial reactants, i.e.

typically in the regime of 1 bar); compensation of low efficiency by large partial

pressures, large media length (typically 1 m), and large pump rate (i.e. in high-

voltage gas discharge, typically in the regime of 20 kV) efficiency (optical

output/electric input) of approx. 1 %

- Possibility of large output powers and repetition rates (e.g. excimers provide very

high-energy pulses in the UV and near-VUV regime), e.g. several 100 mJ per

laser pulse with repetition rates of several 100 Hz

- Small lifetime excited state (approx. 10 ns) large pump rate required.

- laser wavelengths from 108 nm (NeF) to 397 nm (XeF)

- Every pulse of Excimer laser radiation contains a large number of photons, since

it has a very high peak power.

Properties of Excimer laser :

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The Common Excimer lasers

Commercial Excimer lasers can emit (UV) radiation up to an average power of 100 Watts.

• Since the emitted wavelengths are very short, each individual photon carries a large amount

of energy, which is enough to break the bond between molecules in the material that absorbed

the radiation. Thus, the Excimer laser is the perfect cutting tool for almost every material

Special Applications:

• Photolithography - Material processing at a very high accuracy (up to parts of microns !).

• Cutting biological tissue without affecting the surrounding.

• Correcting vision disorders - Cutting very delicate layers from the outer surface of the

cornea, thus reshaping it, to avoid the necessity for glasses.

• Marking on products - Since the short wavelength radiation from the Excimer laser is

absorbed by every material, it is possible with a single laser to mark on all kinds of materials,

such as plastics, glass, metal, etc.

• The price of an Excimer laser is relatively high (tens of thousands of dollars), but it is used a

lot because of its unique properties.

• Pump laser for dye laser systems

Applications of the Excimer Laser:

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The carbon dioxide laser CO2

CO2 has 3 normal modes

of vibration:

The symmetric stretch

(ν1) at 1354 cm-1

The bending vibration

(ν2) at 673 cm-1

The asymmetric stretch

(ν3) at 2396 cm-1.

The CO2 laser is a high power infrared laser of high efficiency that may be pulsed or CW.

It lases between vibrational levels of CO2.

Since the lower state is very short-lived, a population inversion can also be

achieved relatively easily.

m

m

1000

0200

0001

Energy transfer by resonance & collision

Pu

mp

ing

Tra

nsi

tion

s in

CO

2 l

ase

r

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Lasing action in a CO2 molecule was first demonstrated by C. Patel in 1964.

He transmitted an electric discharge pulse through pure CO2 gas in a laser tube,

and got a small laser output.

CO2 is the gas in which the lasing process occurs, but other gas additives

to the laser tube improve the total efficiency of the laser.

The standard CO2 laser includes in the active medium a mixture of CO2 with

N2 and He.

The optimal proportion of these 3 gases in the mixture depends on the laser

system and the excitation mechanism.

In general, for a continuous wave laser the proportions are:

CO2:N2:He ( 1:1:8)

Pumping is achieved by electric discharge – some CO2 molecules are

directly excited, together with efficient transfer from excited N2 to CO2.

Transitions between vibrational levels also involve rotational transitions,

giving rise to a relatively large number of closely spaced emission lines

(the laser can be tuned between these transitions).

Very high laser (up to kW) powers can be achieved.

CO2 is a linear molecule, and the three atoms are situated on a straight

line with the Carbon atom in the middle.

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Various designs of CO2 lasers

1. Longitudinal flow Output Mirror

Power Supply

laser tube(ceramic)

Al sheet

Gas InGas out

Mirror

2. Sealed off- Laser 2CO

He, Ne, CO2 gas mixture

Mirror Mirror

Rf power supply

Laser tube

Laser beam

3. Waveguide 2CO lasers

Rf Power Supply

Insulator

(Waveguide reflectors)

Output mirror

Rear mirror

Metallic electrodes

Waveguide bore

region

Laser

Beam

4. Transverse-flow 2CO lasers

Power supply

Gas flow

Electrode

Output Mirror

Rear Mirror

Laser Beam

Electrode

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5. Gas dynamic Laser 2CO

100kWCan produce powers in the range of

Diffuser

Combustionchamber

Nozzles

Laser beam

High output power. Commercial CO2 Lasers produce more than 10,000

watts continuously.

Output spectrum is in the Infra-Red (IR) spectrum: 9-11 [mm].

Very high efficiency (up to 30%).

Can operate both continuously or pulsed.

Average output power is 75 [W/m] for slow flow of gas, and up to few

hundreds [W/m] for fast gas flow.

Very simple to operate, and the gasses are non-toxic.

Properties of CO2 Laser