Laser Presentation

LASERL IG HT A MPL IFICATIO N B Y S T IMUL ATED E MISSIO N O F R A DIAT ION

SUB MIT ED B Y: MA K WA NA HA R DIK NI .T DE PA R T MENT R O L L NO.: 40

Submitted by: Makwana Hardik N (IT-40)

Simulated Emission

Amplification

Active Medium

Population Inversion

PumpingOptical Resonator

Spontaneous Emission

Q-1: Components of Lasers:

Spontaneous Emission


It is a process in which there is an emission of photon whenever an atom transits from a higher energy state to lower energy state

h

E1

E2

E1

E2

E2 – E1

Before Emission After Emission

The rate of change of spontaneous emission of radiation R21 (sp) is proportional to the population N2 at the higher energy level E2 . i.e.

R21 (sp) α NR21 (sp) = A21 N1

Where, A21 = Probability of spontaneous emission per unit time

WITHOUT the aid of any external agency.

Simulated Emission


It is a process in which there is an emission of photon whenever an atom transits from a higher energy state to lower energy state WITH the aid of any external agency.

h

E1

E2

E1

E2

E2 – E1

Before Emission

The rate of change of spontaneous emission of radiation R21 (sp) is proportional N2 at the higher energy level E2 and to the density ρ of the inducing photon. i.e.

Where, B21 = Probability of simulated emission per unit time

Induced Photon (h)

After Emission

R21 (sp) α N2 ρ

R21 (sp) = B21 N2 ρ

Amplification


It is a process in which there is an emission of photon whenever an atom transits from a higher energy state to lower energy state WITHOUT the aid of any external agency.

The Active Medium


A medium in which population inversion is achieved for laser action is called active medium. The medium can be solid, liquid, gas and plasma.Based on the active medium and method of pumping, the laser are classified into:

1). Solid state lasers

2). Liquid laser

3). Gaseous laser

4). Dye laser

5). Semi-conductor laser

Population Inversion


Population Inversion is a state of achieving more number of atoms in the excited state compared to the ground state.

i.e.N2 > N1

If this condition is satisfied, then there is more chance for simulated emission to take place. Hence, population inversion is an essential condition for producing laser.

It can be achieved by a process called pumping.

Pumping Source


Pumping is the mechanism of exciting atoms from the lower energy state to a higher energy state by supplying energy state by supplying energy from an external source.

Most commonly used pumping is:

1). Optical Pumping: In this type of pumping atoms are excited by means an external optical source.

2). Electrical Pumping: In this type of pumping the electron are accelerated to a high velocity by a strong electric field.

These moving electro collide with the neutral gas atom and ionized the medium. Thus due to ionization they get raised to higher level.

3). Direct Conversion:In this type of pumping, a direct conversion of electric energy into light takes place.

Optical Resonator


It is a pair of reflecting surface (mirrors) of which one is a perfect reflector and the other is a partial reflector.

It is used for amplification of photon there by producing an intense and high coherent output.

Q-2: Nd: YAG laser


Active Medium

This is the four level solid state laser system. Yttrium Aluminum Garnet (Y3Al5O12), Commonly known as YAG, doped with neodymium ion Nd3+ is taken in the form of a crystal and is drawn into a rod. The neodymium ions Nd3+ are the active centers.

Resonator Cavity

The ends faces of the Nd: YAG rod are ground polished and silvered to act as the optical resonator mirrors, or the optical cavity can be formed by using two external reflecting mirrors M1and M2 .

Optical Pumping

A xenon flash lamp or a krypton flash lamp is used as a pumping source.

Construction


A Nd: YAG rod and a krypton flash lamp are enclosed inside an ellipsoidal reflector. In order to make the entire flash radiation to focus on the laser rod, Nd: YAG rod is placed at one focal axis and the flash lamp at other focal axis of the ellipsoidal reflector.

Working


Nd: YAG is a four level laser system. The pumping of neodymium (Nd3+) ions to upper state (E4) is done using krypton arc lamp. Thus optical pumping is used in this laser. The wavelength of light of wavelength 7200 Å to 8000 Å excites the ground state (E1) Nd3+ ions to E4 states. From E4 states, they make a nonradioactive transition and come to E3 state. E3 is the Metastable state so population inversion is achieved between the levels E3 and E2.

After this, the process of stimulated emission will occur. (I have already explained the process of stimulated emission in detail in my previous articles).Thus, the laser emission will occur in between the levels E3 and E2 with the process of stimulated emission. So E3 is the upper laser level and E2 is the lower laser level. Then Nd3+ ions come back to the ground state E1. Laser emission will have wavelength of 10600 Å so occur in the infrared range of spectrum.

Q-3: CO2 laser


Active Medium


Resonator Cavity

The ends faces of the Nd: YAG rod are ground polished and silvered to act as the optical resonator mirrors, or the optical cavity can be formed by using two external reflecting mirrors M1and M2 .

Optical Pumping


Construction


Working


Pumping of nitrogen molecules: As electric discharge is used as pumping source and when electric discharge is passed through the mixture

of CO2, N2 and He, electrons are accelerated down the tube. These accelerated electrons collide with the N2 molecules and excite them to higher vibrational energy levels. Let us say the N2 excited from level F1 to F2.

The level F2 happens to be Metastable and thus the N2 molecules excited to F2 spend a sufficient amount of time before getting de-excited.

Achievement of population inversion of CO2 molecules: The excited level (Metastable level) of CO2 molecules corresponds approximately to the same energy as

the energy of the excited level F2 of nitrogen. Thus when N2 molecules in level F2 collide with the CO2 in the ground state E1, an energy exchange takes place and this result in the excitation of CO2 molecules to level E5 and de-excitation of N2 to the ground level F1.

Thus population inversion is achieved between vibrational levels E5 and E4 or E5 and E3. Thus E5 is the upper laser level. E3 and E4 are lower laser levels.

Q-4: Semi-conductor laser


Active Medium


Resonator Cavity

The ends faces of the Nd: YAG rod are ground polished and silvered to act as the optical resonator mirrors, or the optical cavity can be formed by using two external reflecting mirrors M1 and M2 .

Optical Pumping


Construction


A Nd: YAG rod and a krypton flash lamp are enclosed inside an ellipsoidal reflector. In order to make the entire flash radiation to focus on the laser rod, Nd: YAG rod is placed at one focal axis and the flash lamp at other focal axis of the ellipsoidal reflector.

Working


The flash lamp is switched on. The optical pumping excites the Nd3+ ions from the ground energy level E3 and E4 by absorbing radiation of wavelength 0.08 µm and 0.73 µm, respectively. The energy-level diagram is shown in figure. The excited Nd3+ ions then make a transition from these energy levels. The transition from the

Q-5: Application of lasers


Nd: YAG laser

1). These laser are widely used for cutting, drilling, welding, and surface hardening of the industrial product.

2). These laser are used in military as range finder and as target designation.

3). These laser are used in medical field for cataract surgery, to treat gastrointestinal bleeding and gall bleeding surgery.

4). It is used in long haul communication.

5). It is used in the study of inertial confinement fusion.



C02 laser

1). It is used in material processing such as drilling, cutting, etching, welding, melting, and annealing.

2). It is widely used in open air communication.

3). It is used in pollution monitoring, remote sensing and in LIDAR (Light Detecting and Ranging).

4). It is used in medical field to perform microsurgery and bloodless surgery.



Semiconductor laser

1). These laser are widely used for cutting, drilling, welding, and surface hardening of the industrial product.

2). These laser are used in military as range finder and as target designation.

3). These laser are used in medical field for cataract surgery, to treat gastrointestinal bleeding and gall bleeding surgery.

4). It is used in long haul communication.

5). It is used in the study of inertial confinement fusion.

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