LASER

Light Amplification by Stimulated Emission of Radiation

LASER stands for Light Amplification by stimulated Emission of Radiation. Laser emits light through a process called optical amplification. It based on stimulated emission of electromagnetic radiation.  It is a device that generates light by process known as stimulated emission. Laser light travels in a straight line

To generate laser light, there are three processes. They are:-

• Absorption
• Stimulated emission
• Spontaneous emission

Absorption

The atom in lower energy level absorbs photon energy and move towards the excited state or higher level. This process is known as absorption.

Spontaneous emission

When an electron is excited from lower energy to high energy it cannot stay there. It is in an unstable state. It will come back to the lower state after some time without any external influence. A photon is emitted and its energy will be equal to the energy difference between two levels.  It is called spontaneous emission.

Stimulated emission

When a photon hits on the electron which stays on the higher state, it will stimulate the electron. The photon has the energy equal to the energy difference between two states. The stimulated electron will drop to the lower energy state by emitting an identical photon has the same energy. This process is known as stimulated emission.

Properties of LASER

• Highly monochromatic: - Laser light is monochromatic. That means it contains only one specific wavelength of light. So it is seen in only one colour either red, green etc. It depends on laser.  It can produce more than one colour. But these colours have discrete individual wavelengths of light. The laser is different from sunlight or light from bulbs etc. because it contains different wavelength of light which generally seen as white.
• Highly coherent: -   Due to its high coherent property of laser light, it can deliver the high amount of light as small light beam.
• Highly directional: - The laser beams light are small, tight and bright. They are travelling in a parallel direction.
• Highly focused: - The laser beam of light travels in parallel direction. So it is highly focused than other types of lights. It can deliver high energy over thin space.

Components of a Laser

There are 3 basic components for laser, They are:-

• Lasing material
• Pump source
• Optical cavity

Laser material such as semiconductor, crystal, gas, etc. can be used. Its atoms reside in the lower energy level. Pump source is used to add energy to the laser material such as flash lamp. Optical cavity consists of total reflector and partial reflector and it provides a feedback mechanism which helps for the amplification of light energy or light amplification.

Population Inversion

When the light energy from flash lamp hit on the atoms in the laser material, then the electrons in the atom will excited to higher energy level. If there are more atoms in the higher level than the lower level, then it is not in an equilibrium state. If N1 is the lower state and N2 is the higher state and if N2 is less than N1, then a process known as pumping is used to maintain enough atoms at upper level. Electrical pumping, optical pumping or chemical pumping can be used for pumping purposes. Light is amplified only if N2 is greater than N1. That is N2-N1 is positive. This process is known as population inversion.

But it is an unstable state. After some time the electrons will return back to its original state. It is due to spontaneous emission or due to stimulated emission. In stimulated emission a photon hit on the electron reside in the higher state. This electron return back to the lower energy level by emit an amount of energy equivalent to the incident photon. The direction is parallel to the optical axis causes the emitted photons travel back and forth in the optical cavity. It is through the lasing material in between total reflector and partial reflector. A burst of laser light will be given as output through partial reflector due to this light amplification.

Types of Lasers

• Solid State Laser: - It is a semiconductor laser pumped electrically or optically. Its operation is similar to Light Emitting Diode (LED). Gallium Arsenide is the most commonly used material semiconductor laser. So it is also called Gallium Arsenide Laser or injection laser. When the current is passed through n to p material, the light emits at the junction. Otherwise the current is injected through the junction of n and p material.
• Gas Laser:- A gas tube is placed in laser cavity. The atoms in the gas are excited by applying a voltage. The laser output is taken and this output is normally continuous wave.
• Metal Vapor Laser:- The active medium in this laser is vapor. It consists of metal atoms. An ion laser based on vaporization of solid or liquid metal vaporized with a buffer gas.  

He-Ne Laser

Pumping source will apply energy to the He Ne gas mixture. The Helium atoms move th the excited state. The movement of Helium atom causes the collision of Helium atom and Neon atom. This collision causes the transfer of energy from Helium to Neon and it will excited to the meta-stable state. After achieving a sufficient number of atoms in the meta-stable state, then population inversion occurs. It will lead to the lasing action.

Laser Applications

• Laser Surgery: - The small intense beam of light is focused using lens and it can be used to burn the living tissues.
• Welding, Cutting and Blasting:- It is used for welding cutting etc. It is extremely precise. Two different types of metals or alloys can be easily welding using laser welding. It is better than other techniques.
• Information Technology:- Using optical fiber, a large amount of information can be transmitted by modulated laser beams. Eg:- internet. Also it is used in compact disc players and video disc players. The lasers in players can read the minute reflections in CD.
• Laser Shows.
• Scientific Research:- Laser used in several applications such as atmospheric remote sensing, Laser Induced Breakdown Spectroscopy etc. LIBS is a technique for real time elemental analysis of wide range of materials. Also it is used to monitor the presence of impurities. It is technically similar to the laser based analytical techniques.