Basic components of a laser
(i) Active material or medium
It is the basic material in which atomic or molecular transitions take place, leading to the
laser action. It may be a solid, liquid, gas, dye or semiconductor. A Medium in which
population inversion can be achieved is called an active medium.
(ii) Pumping System
It is a device with which population inversion can be achieved in an active material
(iii) Optical Resonator
An Optical Resonator consists of a pair of reflecting surfaces of which one is fully
reflecting and the other partially reflecting.
The active material is placed between the two surfaces. The photons generated due to
transitions between the energy states of active material are bounced back and forth between the
two reflecting surfaces, the inducing more and more stimulated transition leading to laser action
Fig .9.
R1
R2
Active
medium
100 %
Mirror
Partial
d
Mirror
Fig .9 Optical Resonator
Different types of lasers
Introduction
There are many different types of lasers. The laser medium can be a solid, gas, liquid or
semiconductor. Lasers are commonly designated by the type of lasing material employed:
Solid-state lasers have lasing material distributed in a solid matrix (such as the ruby or
neodymium: yttrium-aluminum garnet "YAG" lasers). The neodymium-YAG laser emits
infrared light at 1,064 nanometers (nm). A nanometer is 1x10-9 meters.
Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have a primary
output of visible red light. CO2 lasers emit energy in the far-infrared, and are used for cutting
hard materials.
Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic
devices are generally very small and use low power. They may be built into larger arrays, such as
the writing source in some laser printers or CD players.
Types of Lasers
Laser systems are generally classified on the basis of active material used as follows:
1. Solid State Laser (Example, Nd:YAG laser)
2. Gas Laser (Example, CO2 laser)
3. Semiconductor Laser (Example, Diode laser)
Nd - YAG Laser
Nd - YAG Laser is a neodymium based laser. Nd stands for Neodymium (rare earth
element) and YAG stands for Yttrium Aluminium Garnet which is (Y 3 Al 4O
12).
It is a four -
level solid state laser.
Principle
The active medium (Nd - YAG rod) is optically pumped by krypton flash tubes and
neodymium ions (Nd
3+
) are raised to excited levels. During the transition from metastable state
to ground state, laser beam of wavelength 1.064 m is emitted.
Construction
The construction of Nd - YAG laser is shown in fig 4.10. In the active element (Nd YAG crystal), a small amount of Yttrium ions (Y 3+) is replaced by neodymium ions (Nd3+). The
active element is cut into a cylindrical rod. The laser rod and a pumping source (flash tube) are
placed inside a highly elliptical reflector cavity. The flash tube is controlled with the help of a
capacitor.
Fig .10 Nd - YAG Laser
The ends of the rod are polished and made optically flat and parallel. The optical
resonator is formed by using two external reflecting mirrors.. One mirror is made 100 %
reflecting while the other is partially reflecting. The system is cooled by air or water circulation.
Working
The energy level diagram for Nd - YAG is shown in fig. 10. The energy levels are those
of neodymium (Nd 3+) ions.
an optically pumped solid-state laser
that can prod very emissions. This is
Fig .11 Energy level diagram of Nd - YAG Laser

When the krypton flash lamp is switched on, the neodymium atoms are raised form the
ground level E0 to upper levels E3 and E4 (Pump band) by the absorption of radiation of
wavelength 0.73 m and 0.80 m.

The neodymium atoms make a transition from these energy levels to level E2 by non radioative transition. E2 is a metastable state.

The neodymium ions are collected in the level E2. Population inversion is achieved
between E2 and E1.

An ion makes a spontaneous transition from E2 to E1, emitting a photon of energy hv.
This emitted photon will trigger a chain of stimulated photons between E2 and E1.

Photons thus generated travel back and forth between the two mirrors and grow in
strength. After some time, the photon number multiplies more rapidly.

After enough strength has been attained (condition for laser satisfied), an intense laser
light of wavelength 1.06 m (10600 Å) is emitted through the partial reflector. It
corresponds to the transition from E2 to E1.

The transition from E1 to E0 is rapid and non - radiative.
Characteristics

Type : It is a four - level solid state laser

Active medium : The active element is a Nd - YAG crystal (rod).

Pumping method : Optical pumping is employed for the pumping action.

Pumping source : Xenon or krypton flash tube.

Optical Resonator : Two ends of ND - YAG rod polished with silver (one end fully
silvered and the other is partially silvered) are used as optical resonator.

Power Output : The Power output is approximately 70 watt.

Nature of Output : The nature of output is pulsed or continuous beam of light.

Wavelength of output : The wavelength of the output beam is 1.06 m (infra - red).
Advantages
1.
It has high energy output.
2.
It has very high repetition rate operation.
3.
It is much easy to achieve population inversion.
4.
YAG is a crystalline material, and hence, the corresponding line width is
much smaller, which implies much lower thresholds.
Disadvantages
The electron energy level structure of Nd 3+ in YAG is complicated.
Applications
1.
It finds many applications in range finders and illuminators.
2.
It is widely used in engineering applications such as resistor trimming, scribing,
micro - machining operations as well as welding, drilling etc.
3.
It finds many medical applications such as endoscopy, urology, neurosurgery,
ENT, gynaecology, dermatology, dental surgery and general surgery.
Carbon Dioxide (CO2) Laser
It was the first molecular laser developed by an Indian born American Scientist Prof.
C.K.N.Patel.
It is a very efficient laser. It is a four - level molecular gas laser and operates at 10.6 m
in far IR region.
Energy states of CO2 molecules
A carbon dioxide molecule has a central carbon atom with two oxygen atoms attached,
one at both sides. Fig 13.3. Such a molecule exhibit three independent modes of vibrations.
The energy states of the CO2 molecule arise due to its vibrations .
Fig .12 Different modes of vibrations of CO 2 molecule
(i). Symmetric stretching mode
In this mode of vibration, the carbon atom is at rest in its position and both oxygen atoms
vibrate simultaneously along the axis of the molecule departing or approaching the fixed carbon
atom Fig 12.
(ii). Bending mode
In this mode of vibration, the oxygen atoms and carbon atom vibrate perpendicular to the
molecular axis Fig 12.
(iii). Asymmetric stretching mode
In this mode of vibration, the oxygen atoms and carbon atom vibrate asymmetrically, i.e.,
oxygen atoms move in one direction while carbon atom moves in the opposite direction Fig 12.
Principle
The active medium in CO2 laser is CO2 gas. Laser transition takes place between the
vibrational states of CO2 molecules.
Construction
Fig 13 Schematic diagram of CO2 laser
The CO2 laser in Fig 13. consists of a quartz discharge tube 4m long and 2.4cm in
diametre. The discharge tube is filled with gaseous mixture of CO2 (active medium), helium and
nitrogen with suitable partial pressures.
The terminals of the discharge tube are connected to a D.C Power supply. The ends of the
discharge tube are fitted with NaCl Brewster windows so that the laser light generated would be
polarized.
The concave mirrors are provided, one completely reflecting and the other partially
reflecting. They form the optical resonator.
Working
Fig 14.shows the vibrational level of the electronic ground state of CO2 and N2
molecules.
When an electrical discharge is passed through the gas, the electrons collide with nitrogen
molecules and are raised to excited states. This is represented by
N2 + e
N* 2 + e1
N2
is the nitrogen molecule in ground state
e
is the electron with kinetic energy
N* 2
is the nitrogen molecule in excited state.
e1
is the same electron with lesser energy.
Fig 14 Energy level diagram of CO2 laser
Subsequently N2 molecules in excited state collide with CO2 atoms in ground state and
excite them to higher electronic, vibrational and rotational levels.
N*2 + CO2
CO*2 + N2
N*2
is the nitrogen molecule in excited state
CO2
is carbon dioxide atoms in ground state
CO*2 is carbon dioxide atoms in excited state.
N2
is the nitrogen molecule in ground state.
Since the excited level of nitrogen is very close to the 001 level of CO2 atom, the
population in 001 level increases. As soon as population inversion is reached, any of the
spontaneously emitted photon will trigger laser action in the tube. There are two types of laser
transition possible.
001 - 100 transition : This transition would produce a laser beam of wavelength 10.6 m.
001 - E3 transition : This transition would produce a laser beam of wavelength 9.6 m.
Normally 10.6 m transition is more intense than 9.6 m transition. The power output
expected from this laser is 10 kW.
The helium gas is used in conducting the heat generated in the central region of the
discharge tube to the walls of the discharge tube.
Characteristics

Type : It is a molecular gas laser

Active medium : A mixture of CO2, N2 and helium or water vapour is used as active
medium.

Pumping method : Electrical discharge method is employed for the pumping action.

Optical Resonator : Two concave mirrors form the resonant cavity..

Power Output : The Power output expected from this laser is about 10 kW.

Nature of Output : The nature of output may be continuous wave or pulsed wave.

Wavelength of output : The wavelength of the output beam is 9.6 m and 10.6 m
(96000 Å and 10600 Å).
Advantages

The construction of CO2 laser is simple.

.The output of this laser is continuous.

It has extremely high efficiency.

It has very high output power.

The output power can be increased by extending the length of gas tube.
Disadvantages

The contamination of oxygen by carbon monoxide will have some effect on
laser action.

Operating temperature plays an important role in determining the output
power of laser.

Corrosion may occur in the reflecting plates.

Accidental exposure may damage our eyes, since it is invisible to our eyes.
Applications

High - power CO2 laser finds application in materials processing, welding,
drilling, cutting, soldering, etc.

The low atmospheric attenuation (10.6 m) makes CO2 laser suitable for
open air communication.

It is used in remote sensing.

It is used in the treatment of liver and lung diseases.

It is largely used in neurosurgery and general surgery.

It is used to perform microsurgery and bloodless operations.