Popular Pumping Mechanisms

Requirements of a LASER

(i) Active Medium :- Gain per unit length is an inherent property.

(ii) Optical Resonator :- Length (L) dependence of gain ( but L can not

be very very high without limit, rather it should be as low as

possible).

(iii) Pumping Mechanism (Method of obtaining Population Inversion) :-

Has to be very effective in order to ensure high enough “Population

Difference”.

NOTE :- All three of the above are equally important. However, having

chosen an active medium and optimized the resonator parameters,

the factor most potentially tailored is the Pumping mechanism.

Population Inversion Methods / Pumping Mechanisms

(i) Optical Pumping

(ii) Electrical Pumping

Other Methods

(iii) Chemical Pumping

(iv) Gas dynamic Pumping

(v) Laser Pumping

(vi) Nuclear Pumping

(vii) Particle-kinetic energy Pumping

} Most common

Ruby (Al2O3 – Cr3+)

Nd:YAG (Nd –Y2Al5O12), Yb:YAG, Er: YAG

Nd:Glass, Er:Glass

Nd:KGW

Nd:YVO4

Nd:GSGG

Nd:YLF

Ti:Sapphire, Cr:LiSAlF, Cr:LiCaAlF

Alexandrite (Cr doped chrysoberyl = BeAl2O4 – Cr 3+)

Dye Lasers (Liquid Laser)

Cs Vapor (Gas Laser)

Fiber Lasers

Optically Pumped Lasers :-

Solid State

Lasers

Gas Lasers :-

(i) Atomic :- He-Ne, He-Cd, He-Zn, He-Hg, Cu Vapor, Au Vapor, Pb

Vapor, Water Vapor (Far IR – 30 µm to 1.8 mm)

(ii) Ionic :- Ar, Kr, Ne, Xe

(iii) Molecular :- CO2, CO, N2, Excimer, CH3OH, C2H2F2, CH3F, HCN

(Far IR)

Electrically Pumped Lasers :-

Semiconductor Lasers :-

(i) Binary :- Ga As, In P, Zn Se ( II-VI), Ga N (Blue-Green)

(ii) Ternary :- In Ga As, Ga Al As

(iii) Quaternary :- In Al Ga As, In Al Ga P, Ga In As Sb, Al Ga As Sb

Chemical Pumping :-

HF, DF, HCl, HBr, COIL, I-Photodissociation

Gas Dynamic Pumping :- CO2 GDL

Laser Pumping :-

CH3 OH (by CO2 Laser), Nd:YAG (by Diode Lasers),

Dye Laser (by doubled & tripled Nd:YAG, N2, Ar+, Excimer - KrF, XeF,

XeCl, Q-switched Ruby, Copper vapor, Krypton Laser)

Nuclear Pumping :- X-ray Laser

Other Pumping methods:-

Free Electron Laser (Particle-kinetic energy Pumping), X-ray Laser,

Gamma Ray Laser

NOTE :- Gas lasers do not lend themselves so readily to optical pumping

because of the small widths of their absorption lines and usually broad

emission of the pumping lamps.

However, He lamp (~ 390 nm) and Cs vapor absorption lines match

and hence optical pumping of the gas laser is possible.

OPTICAL PUMPING

G

E

ULL

LLL

Optical Pumping

Many optically pumped lasers have a gain

medium consisting of rare earth or

transition metal ions doped into an

insulating dielectric solid.

In a laser that is optically pumped, the

upper laser level is populated by

absorption of a photon from some optical

source.

That is the laser material is illuminated

with light at the right wavelength to excite

the lasing species.

The light source can be a high-intensity

lamp (lamp pumping) or another laser

(laser pumping).

The upper levels of the pump transition usually span a range of energies.

In fact, there are typically multiple upper levels, which all decay to the

metastable ULL.

This means the laser can be excited at many wavelengths corresponding

to any transition between G and those many upper levels. Thus, many

(solid state) lasers are optically pumped with light sources emitting a

broad range of wavelengths.

The early lasers were mostly lamp-pumped.

But the trend in recent years has been toward laser-pumped lasers.

Pumping Process :-

Light from a powerful source (Flash lamp or Arc lamp or incandescent

lamp) is conveyed to the active material which is usually in the form of a

cylindrical rod (diameter of few mms to few cms and length of few cms to

few 10s of cms).

The laser can be operated in pulsed or CW mode depending on whether

the pump source is pulsed or continuous.

NOTE :-

“Optical pumping is a process in which light is used to raise (or ‘pump’)

electrons from a lower energy level in an atom or molecule to a higher

one”.

The technique was developed by 1966 Nobel Prize winner Alfred Kastler in

the early 1950s.

Schematic of Optical pumping of a laser rod (bottom) with an arc lamp (top).

[Red : hot. Blue : cold. Green : light. Non-green arrows: water flow. Solid

colors: metal. Light colors: fused quartz].

Commonly used Optical Pumping Configurations

1. Helical :-

Rod

Lamp

Lamp

Rod

Lamp is a long Quartz tube

coiled into a helix. Diameter of

the helix is small and helix is

wound tightly.

Light reaches directly or after reflection at the specular cylindrical surface.

2. Elliptical / Cylindrical :-

Lamp

Rod

Lamp is in the form of a cylinder (Linear Lamp)

and the length is ~ that of the active rod.

Lamp is placed along one of the focal axes

F1 of the elliptical cylinder and the rod is placed

along the second focal axis F2.

Elliptical reflector

The distance between the electrodes, referred

to as the “arc length” of this lamp, is generally

chosen to be about the same as the laser rod

length.

The “bore” of the flashlamp (the inside

diameter of the quartz tubing or "envelope")

is usually the same as the diameter of the laser

rod. The lamp and rod are placed inside a

reflecting housing with their axes parallel.

Cylindrical reflector

Gas arc lamp with water jacket for cooling

Example :-

Linear single and double lamp pumping of Nd:YAG Laser

Single lamp elliptical reflector cavity

Cylindrical close-coupled

3. Close Coupled Configuration :-

The rod and the lamp are placed as close as possible

and are surrounded by a close coupled cylindrical

reflector. Cylinders made of diffusely reflecting

material (Eg : Compressed MgO or BaSO4 powder or

white ceramic) are often used.

4. Multiple Configurations :-

Double- Ellipse

Close-coupled (Double)

Multiple configurations using more

than one elliptical cylinder or several

lamps are used.

Efficiency of multiple designs

is lower than the corresponding single

configurations, but are used in High

Power systems.

Close-coupled configurations (a) Circular cylinder; (b) Single-lamp close-

wrap; (c) Double-lamp close-wrap (d) Four-lamp close-wrap (e) Close-

coupled multiple coaxial design

Four-lobe Elliptical Spherical

NOTE :-

Helical pumping is simple but efficiency is poor.

A reasonably efficient pumping geometry is an elliptical cylinder reflector.

Greater pumping efficiency is achieved with the rod and lamp as near one

another as possible with an ellipse of low eccentricity.

The most common pumping configurations are single ellipses with one lamp

and double ellipses with two lamps.

Arrangement of Pump and Laser Rod

A ruby laser head

Laser pumping lamps. The top three are xenon flashlamps while the

bottom one is a krypton arc lamp

These gas discharge lamps show the spectral line outputs of the

various noble gases.

NOTE :-

Three sources (lamps) for Optical Pumping :-

(a) Flash lamps (Pulsed pumping)

(b) Arc lamps (CW pumping)

(c) Incandescent lamps (Cheaper CW pumping with a Tungsten

wire)

Flash/Arc Lamps generally use Xenon or Krypton gas inside a Quartz

tube.

The Krypton lamp produces most of its output light in the infrared

region of the absorption bands of Nd:YAG and Nd:Glass. Thus, it is the

best spectral match for these laser materials.

Krypton lamps are not widely used because of their cost (Far more

expensive than xenon lamps).

Xenon lamps usually have lower efficiency. But they have sufficient

output in the desired spectral region & their lower efficiency is usually

acceptable.

Xenon flash lamps have greater emission in the blue-green region of ruby

laser absorption. Thus, they are used with all ruby lasers.

In spite of being expensive, high efficiency requirements and high power

Nd:YAG and Nd:Glass systems demand the use of Krypton lamps.

End pumping

Side pumping

Diode Laser Pumping

Pumping Efficiency :-

To calculate or estimate the pumping efficiency, the pump process can be

divided into four distinct steps :

1. Emission of radiation by the lamp

2. Transfer of this radiation to the active medium

3. Absorption in the medium

4. Transfer of the absorbed power to the upper laser level

Thus, the pumping efficiency ηP can be written as the product of four terms

as follows –

Where, ηr = Lamp radiative efficiency

ηt = Transfer efficiency

ηa = Absorption efficiency

ηpq = Power quantum efficiency

r t a pqP . . . . . . . . . . . (1)

Lamp radiative efficiency (ηr) = The efficiency of conversion from electrical

input to light output in the wavelength range corresponding to the pump

bands of the laser medium.

Transfer efficiency (ηt) = The ratio of the pump power actually entering the

rod to that emitted by the lamp in the useful pump range.

Absorption efficiency (ηa) = The fraction of the light entering the rod that is

actually absorbed by the material.

Power quantum efficiency (ηpq) = The fraction of the absorbed power that

leads to the population of the ULL.

Typical Values :-

ηr = 0.43 for flash lamp pumped Nd:YAG laser

= 0.36 for flash lamp pumped Alexandrite laser

ηt = 0.9 – 0.8 for elliptical pump cavity

= 0.62 for helical lamp

Comparison of computed ηP values :

[ 6.3 mm diameter rod ; Elliptical pump chamber, lamp current density 2000

3000 A/cm2 ; Lamp diameter = 5 mm ]

Material ηr (%) ηt (%) ηa (%) ηpq (%) ηP (%)

Ruby 27 78 31 46 3.0

Alexandrite 36 65 52 66 8.0

Nd:YAG 43 82 17 59 3.5

Nd:Glass 43 82 28 59 5.8

Nd:Cr:GSGG 43 82 54 48 9.1

Summary :-

1. Radiative efficiency is < 50 % in each case.

2. Absorption efficiency for Nd;Cr:GSGG is about 3 times that of

Nd:YAG (because of Cr doping).

3. Absorption efficiency for Alexandrite is quite high.

4. Nd:Cr:GSGG and Alexandrite show high overall efficiency.

Comparison between Lamp pumped and Diode pumped Nd:YAG laser

Nd:YAG pumped by GaAlAs QW laser at 808nm with emission

Bandwidth 1-2 nm.

It can be seen that radiative and transfer efficiency is almost same but

there is very large increase in absorption efficiency which leads to

higher overall pump efficiency.

Example of Dye Laser

Dye Laser

“Stimulated emission observed from an organic dye,

chloroaluminum phthalocyanine” P. P. Sorokin and J. R.Lankard

IBM J. Res.Dev. 10, 162 (1966).

755 µm

Dye Laser

A dye (Liquid) laser is a laser which uses an organic dye as the

lasing medium, usually as a liquid solution. Compared to gases

and most solid state lasing media, a dye can usually be used for a

much wider range of wavelengths.

The wide bandwidth makes them particularly suitable for

tunable lasers and pulsed lasers. Moreover, the dye can be

replaced by another type in order to generate different

wavelengths with the same laser, although this usually requires

replacing other optical components in the laser as well.

Some of the dyes are Rhodamine 6G, fluorescein,

coumarin, stilbene, umbelliferone, tetracene, malachite green.

Setup of a Tunable Dye Laser

Attractions :-

1. Unusual flexibility and Tunability Near UV to Visible and

Near IR

2. Extremely narrow Spectral Bandwidth (Ultrapure light)

3. Ultrashort pulses (ps to ~ 25 fs)

Disadvantages :-

Rapid degradation during operation

Very Complex liquid handling requirement

Limited output power

Need for pumping with green or blue laser, making the

pump sources expensive

Handling of poisonous, often even carcinogenic and dirty

material

Dyes themselves as well as the used solvents are sometimes

highly toxic (A particularly hazardous solvent, sometimes

used for cyanide dyes, is dimethylsulfoxide (DMSO), which

greatly accelerates the transport of dyes into the skin)

Construction

Since organic dyes tend to degrade under the influence of

light, the dye solution is normally circulated from a large

reservoir. The dye solution can be flowing through a cuvette,

i.e., a glass container, or be as a dye jet, i.e., as a sheet-like

stream in open air from a specially-shaped nozzle.

With a dye jet, reflection losses from the glass surfaces and

contamination of the walls of the cuvette are avoided. These

advantages come at the cost of a complicated alignment.

Dye lasers emission is inherently broad. In order to produce

narrow bandwidth tuning there are many types of cavities

and resonators which include gratings, prisms and etalons.

Rhodamine 6G, emitting at 580 nm (yellow-orange).

The most popular dye used for the dye laser is Rhodamine 6G.

The reasons for its popularity :-

Its low cost

Effectiveness

Easy availability

Low toxicity

Using Rhodamine 6G as the dye enables tuning of the

output laser beam’s wavelength between 540 nm to 640 nm,

(peak energy at 590 nm) depending on other factors in the laser.

Dye laser pumped by 532 nm doubled Nd:YAG

Nd:YAG

Laser

Dye Laser

Doubled Nd at 532 nm

Lasers suitable as pump source for Dye Lasers

Nitrogen (N2)

Argon Ion

Q-switched Ruby

Copper vapor

KrF

XeF

Frequency doubled Nd:YAG

Frequency tripled Nd:YAG

XeCl

Krypton

Working of Dye Laser

The laser cycle begins with

the dye molecule in the S0

level. A photon is absorbed,

raising the molecule to some

vibrational and rotational

energy in the S1 level.

The molecule very quickly

undergoes an electronic

transition (in a few femtoseconds), so that the molecule settles

into the S1,G level. This transition does not produce laser

radiation.

(Intersystem crossing)

Excited state (S2)

Triplet state (T2)

Triplet state (T1)

There are five paths by which the dye molecule may leave the

S1,G state. These are :-

Spontaneous emission

Stimulated emission

Excited-state absorption of a pump photon

Excited-state absorption of a laser photon

Decay into a triplet band

Only stimulated emission produces a usable laser beam. The

other processes generally only reduce the amount of usable

energy for the output beam and increase the heating of the

solvent.

states from which laser emission takes place are called Singlets

(S). The Triplet states (T) do not contribute to lasing process.

Absorption of radiation takes the molecule from the

bottom level of S0 to one of the S1 levels, where non-radiative

decay quickly brings it to the bottom level of S1. The later serves

as the ULL, the LLL being one of the Vibrational-Rotational

levels of S0.

Although S T transitions are radiatively forbidden,

they may occur non-radiatively in collisions between molecules.

Eg :- S1 T1 , T1 S0 . (Intersystem crossing)

Lasing begins when incident energy is absorbed by the dye,

exciting it from the lowest singlet state to a high-energy level

within the upper singlet band.

From the high-energy level the dye falls to a slightly lower

state within the same singlet band, which serves as an upper

lasing level.

A laser transition can then occur between the upper lasing

level and the lower singlet state, which serves as a lower lasing

level.

NOTE :-

An alternative pathway exists to destroy laser action in the

triplet states of the dye.

NOTE :

Triplet states originate when excited electrons in the dye

molecule spin in the same direction as that of the remaining

electrons in the dye molecule.

The singlet states result when the excited electron spins in the

direction opposite to the lower-energy-state valence electrons

still in the dye molecule.

Because triplet states have lower

energies than corresponding

singlet states, dye molecules can

easily migrate to those states

and in doing so depopulate the

upper lasing level.

Thus, Triplet quenching is required for efficient operation of

Dye lasers. This is done by either

(a) Rapidly flowing the dye

(b) Using a pump source with a short pump pulse (e.g., N2 laser

with 10 ns pulses)

(c) Adding triplet quenching additives like cyclooctatetraene.

They provide deexcitation pathway; dye molecules re-enter.

NOTE :-

Triplet states are metastable and have much longer lifetimes

than the singlet levels.

When a flashlamp is used (generally have pulse widths of over

1 ms), triplet states can form. For this reason, flashlamps

must be designed to discharge as quickly as possible.

Pumping Configurations :-

1. Longitudinal/End Pumping

Dye laser cavity is collinear with the pump laser cavity.

Pump Laser Output

Mirror Mirror

Dye Cuvette

Longitudinal Pumping Configuration

2. Transverse Pumping

The axis of the dye cavity is perpendicular to the axis of the

pump laser cavity.

Pump Laser

Output

Mirror

Mirror

Dye Cuvette

Transverse Pumping Configuration

Cylindrical

Lens

Schematic of Laser-pumped (transverse) dye laser

590 nm

Problems :-

The biggest problems in Dye lasers are –

(a) The heat management

(b) Degradation of the dye itself

Solution :-

Both problems are alleviated by forming the dye into a

continually flowing sheet of liquid called a laminar flow.

Flowing dye is pumped through a nozzle to create a broad, flat

stream onto which pump laser light is focused by a lens.

NOTE :- Dye flow helps suppress the effects of triplet absorption

in the dye by ensuring a fresh supply of dye .

ELECTRICAL PUMPING

Electrical Pumping

Achieved by allowing a current to pass through the gas mixture. Generally,

the current through is passed either along the laser axis direction

(Longitudinal discharge) or transversely to it (Transverse discharge).

An electric discharge may be produced in a gas contained inside a glass tube

by applying a high voltage to the electrodes on either side of the tube.

Electrons are ejected from the cathode and drift towards the anode. When

an electron collides with an atom (or molecule), there is a probability of

raising it to some higher energy state.

Discharge Process :- In an electrical discharge, ions and free electrons acquire

additional KE from the applied electric field and are able to excite a neutral

atom by collision.

NOTE :- The positive ions, owing to their much heavier mass, are accelerated

to lower velocities and thus do not play any significant part in the

excitation process.

Electrical pumping occurs via one or both of the following processes –

(i) Electron Impact (Direct) Eg: N2 Laser

(ii) Resonant Energy Transfer (Indirect) Eg: He-Ne Laser, CO2 Laser

Electron Impact

*e X X e

Here, the gas consists of only one species. X is the atom in the ground state

and X* is in excited state.

This is called “Collision of the 1st kind”

The electron loses KE. Energy lost by the electron is converted to internal

excitation energy of the atom.

Total energy (Internal + KE) before and after the collision are the same.

The internal energy added to the molecule may be in the form of vibrational

and rotational energy, as well as electronic energy.

Resonant Energy Transfer

For a gas consisting of two species (A & B), the excitation can occur as a

result of collisions between atoms of different species.

Let B be in the ground state and A be in the excited state brought about by

electron impact. After the collision, species A will be in ground state and B

in excited state -

* *A B A B E

The energy difference ΔE will be added to or subtracted from the

translational energy.

This is called “Collision of the 2nd kind”.

This is an attractive way of pumping B, if the upper state of A is

metastable forbidden transition.

Hence, once A is excited to its upper level, it will remain there for a long

time, thus constituting an energy reservoir for excitation of the species B.

Resonant Energy Transfer Process

An excited species can transfer energy to another by photon transfer. That

is, the photon spontaneously emitted by one species is absorbed by the

other.

Here, the 1st species drops to a lower level and the 2nd species is raised to a

higher level. This means there is an excitation transfer.

Hence, one requirement is : The photon emitted by the donor species must

be within the absorption linewidth of the acceptor species, i.e., there must

be a resonance (or near-resonance) of the atomic transitions.

The transfer cross-section is large, when the corresponding atomic or

molecular transition frequencies are approximately equal. However,

excitation transfer can occur between species A and B even if the

transitions are not precisely resonant.

The energy defect (ΔE) can be made up from translational degrees of

freedom, thus in accordance with the Law of Conservation of Energy.

Based on whether the temperature of the system is raised or lowered, the

energy defect can be of two types :-

(i) Positive energy defect (Exothermic)

(ii) Negative energy defect (Endothermic)

Positive energy defect : A* + B = A + B* + ΔE

ΔE

A*

A

B

B*

This process raises

the temperature of

the system

There is Exothermic excitation transfer. The extra energy ΔE after the

excitation transfer appears as additional KE of A & B.

Negative energy defect : A* + B = A + B* - ΔE

This endothermic process lowers the temperature of the system AB. The

defect in energy is made up for at the expense of the collision partners. The

KE of AB after the excitation transfer is less than that before the transfer.

ΔE A*

A

B

B*

Example of CO2 Laser

CO2 Laser

Lasing in a CO2 molecule was first demonstrated by CKN

Patel in 1964.

CO2 is the active gas in which the lasing process occurs.

The standard CO2 laser includes in the active medium a

mixture of CO2 with N2 and He. The optimal proportion of

these three gases in the mixture depends on the laser

system and the excitation mechanism. Generally, for a

continuous wave laser the typical proportions are:

CO2:N2:He - 1:1:8

CO2 is a linear tri-atomic molecule, and the three atoms are

situated on a straight line with the Carbon atom in the middle.

Three vibrational modes of CO2 molecule are illustrated :

Symmetric stretch mode

Normal mode frequency 1388 cm-1

Asymmetric stretch mode

Bending mode

Normal mode frequency 667 cm-1

Normal mode frequency 2349 cm-1

N2 (V = 0)

Lasing in CO2 laser occur when there is a transition from higher energy

level of the asymmetric mode into one of the other two.

The transition to the symmetric stretching mode correspond to the

wavelength of 10.6 μm (The most powerful and popular line).

The transition to the bending mode correspond to ~ 9.6 μm.

001

100 020

010

V0

V1

Electric discharge is created in the laser tube. The energy of the accelerated

electrons is transferred by collisions to the Nitrogen molecules and CO2

molecules.

– Nitrogen molecules help in the process of the excitation of the CO2

molecules. The first vibrational energy level of the Nitrogen molecule is very

similar to the asymmetric stretching mode of the CO2 molecule, so energy can

be easily transferred from the excited Nitrogen molecules to the CO2

molecules.

Helium molecules are added to the gas mixture in order to:

– Empty the lower laser energy level so that population inversion is maintained.

– Stabilize the electrical discharge by taking heat away from the lasing area.

Gas pressure inside the CO2 laser tube is 5-30 [Torr], of which 10% CO2 gas,

10% N2 and the rest is He.

Note :- The specific heat (which determines the thermal conductivity) of He [1.24

cal/gr* 0K] is five times that of Nitrogen [0.249 cal/gr* 0K].

Role of N2 and He in CO2 Laser Operation