This article has been authored by Prashant Sharma, a Technology Columnist-Engineer by Profession who has scripted a cluster of Utility focussed and simplistic write-ups relating to Physics out here.His fundamental striking articles at XAmplified are easy-to-understand and helpful enough for Physics-phobic folks
Table Of Content
LASER is an abbreviated form of “Light Amplification by Stimulated Emission of Radiation”. It is an instrument which produces coherent and monochromatic light beam, which is uniform in nature and can be focused to very small and far off objects as well. Laser produces either pulsed or continuous output and has a number of applications in various fields.
There is an important phenomena related to LASER called ‘Population Inversion’ which has been described below.
There are a number of techniques for pumping a collection of atoms to an inverted state. Most common method of pumping is optical pumping in which the atoms are raised by illumination of a strong source of light say a flash lamp. In other words, if luminous energy is supplied to medium for casing population inversion, then the pumping is called the optical pumping. In pumping the luminous energy usually comes from a light source in the form of short flashes of light.
This method was first used in Ruby Laser and is now as days in solid state lasers. The laser material is simply placed inside a helical xenon flash lamp of the same type as used in photography.
Types of Laser
The laser is broadly classified as :
- Gas Laser
- Semiconductor Laser
- Solid state Laser
- Chemical Laser
- Dye Laser
- Metal vapour Laser
Out of the above mentioned types , the most commonly used laser types are Gas Laser(He-Ne Laser) and Semi-conductor Laser.
Helium Neon Laser
The He-Ne laser was the first laser to employ as gaseous medium operated successfully by Ali and his co-workers at Bell Telephone Laboratories in U.S.A. in 1961. This is the first laser of any kind in which continues laser action was demonstrated.
A mixture of about a parts of helium and I part of neon (He : Ne = 9 : 1 ) is contained in a glass tube at a pressure of about 1 mm of mercury. The ends of the tube are fixed with two optically plane and parallel mirrors, one partially silvered and the other perfectly silvered. The spacing of the mirrors is equal to integral number of half wavelengths of the laser light. An electric discharge is produced in the gas mixture by electrodes connected to a high frequency electric source.
The electron from the discharge pumps the helium atom the ground state to a stable at energy of 20.61 eV. The excited helium atom now collides in elastically with neon atom and transfers its energy to it. Incidentally the neon atom has got excited state at the energy level 20.66 eV, the 0.05 eV of additional energy being provided by the kinetic energy of atom after collision. Thus the neon atom is raised to this level and the helium atom, after transferring its energy to the neon goes back to its ground state. Thus he atoms helps in achieving population inversion for the Ne atoms.
Below the metastable state, the neon atom has another lived excited state at the energy level of 18.70 eV. When an excited Ne atom passes spontaneously from the metastable state at 20.66 eV to an excited state at 18.70 eV, it emits a 6328Ao photon travels through the gas mixture, and if it is moving parallel to the axis of tube, is reflected back and forth by the mirror ends until it stimulates an Ne atom and cause it to emit a 6328 Ao photon in phase with the stimulating photon. This stimulating transition from 20.66 eV to 18.70 eV level is the laser transition. This process is continued and a beam of coherent radiations builds up in the tube. When this beam becomes sufficiently intense, a portion of it escapes out through the partially silvered end.
From 18.70 eV level, the Ne atom passes down spontaneously to a lower metastable state of energy 16.70 eV emitted in coherent light and finally to the ground state through collision with the tube walls. Thus the final transition is radiation less.
Advantage of He-Ne laser
It is a gas laser therefore; crystal and other imperfections in the solid which lead to slight beam divergence and slight spread of wavelength are avoided in gas laser. Unlike the pulsed excitation from the Xenon lamp in the Ruby laser operate continuously.
The semiconductor lasers are similar to others as the emitted radiations are highly monochromatic and directional on important difference is solid state and semiconductor lasers in that in the former only 1% of the working surface is active while in the letter the whole material is active.
- The semiconductor be such that the transition probability for a radiative transition across the conduction and value gap must be high and must exceed the probability for non – radiative transfer of energy to the lattice etc.
- The excess population can be maintained across the laser transition.
- The laser beam, narrow, intense, parallel monochromatic and highly coherent is used in various fields.
- In the technical field, the laser beam is used for cutting steel sheets and melting and drilling hard materials. It can create hole in diamond.
In the medical field, the laser beam is used in delicate surgery as cornea grafting with laser beam the surgery is completed in much shorter time. The laser beam is also used in the treatment of kidney stone, cancer and tumour and in depositing and cutting the blood cells in brain operations.
- During was-time lasers are used to detect and destroy the enemy missiles.
- Laser is very useful in science and research. It has been used to perform Michelson-Morley experiment which is the building stone of the Einstein theory of relatively.
- Laser is used for three dimensional photography.
- Since laser rays are having narrow angular spread so these are used in measuring very long distances.
- Laser rays have proved to be useful in detaching nuclear explosions and earth quakes, in vaporizing solid fuel of rockets.