LASER (Light amplification by stimulated emission of radiation)Fundamental to applications

The laser light produces an intense, concentrated, and highly parallel beam of coherent light.  Laser is the outgrowth of maser, a similar device using radio microwaves instead of visible light waves. Some basic principles involved in the operation of laser. Meta stable states, optical pumping, fluorescence, population inversion, resonance, stimulated emission, coherence, polarization, fabry-perot interferometry, cavity oscillation. Most of the principles are already used in some other applications but coherence and stimulated emission are the keys to the realization of maser and laser operation.

Consider a gas enclosed in a vessel containing free atoms having several energy levels, at least one of which is meta-stable. White light into these gas atoms can be raised from the ground state to the excited state. If the light intensity is high, we can obtain more electrons in excited states which means population inversion. When an electron in one of these excited states spontaneously jumps to the ground state, as it eventually will, it emits a photon of energy hv. This is called fluorescent or phosphorescent radiation. As the photon passes by another nearby atom in the same Meta stable state, it can, by the principle of resonance, immediately stimulate that atom to radiate a photo of the exact same frequency and return it to its ground state. That stimulated photon has exactly the same frequency, direction, and polarization as the primary photon and exactly the same phase and speed.

            Both of these photons may now be considered primary waves, and upon passing close to other atoms in their meta-stable states, they stimulate them to emission in the same direction with the same phase. However, the transition from the ground state to the excited state can also be stimulated, thereby absorbing the primary wave. An excess of stimulated emission therefore requires a population inversion, more atoms in the Meta stable state than the ground state. Thus if the conditions in the gas are right, a chain reaction can be developed, resulting in high-intensity coherent radiation.

The Ruby laser

The ruby is primarily a transparent crystal of corundum doped with trivalent chromium ions. The chromium ions are the active ingredients. The aluminium and oxygen atoms are inert. The ruby crystal is cylindrical in shape. Generally ruby laser one end is highly reflective and the other end is close to half silvered. When white light enters into a crystal, absorption by the chromium ions in the blue-green part of the spectrum occurs. The electrons immediately rise excited state and randomly jump back to the ground level, emitting visible red light.

Helium-neon gas laser

The helium-neon gas laser was the first gas laser operation. Many scientists have tried to make gas lasers because it is inexpensive, unusually stable, and emit continuously, this helium-neon laser is widely used in optics laboratories. There are ten times as many helium atoms present in a mixture as there are neon atoms, the orange colour of the gaseous discharge is characteristic of neon atoms. The visible spectrum of helium contains lines in red, yellow, green and blue, so the discharge appears as white light.

The carbon dioxide laser

A high-power molecular gas laser operates on carbon dioxide gas molecules. This optical device produces a continuous laser beam with a power output of several kilowatts and at the same time maintains a relatively high degree of purity and coherence.

At present hundreds of different kinds of lasers using many different materials have been made, emitting radiation over a wide range of wavelengths from the ultraviolet at one end of the spectrum to microwave at the other. Modulated laser beams have been used for communication. The modern medical industry depends upon lasers and in future all technology will have some kind of laser. Laser communication is used to communicate small and large distances. Lasers have been used like radar, and determine distances. Lasers are also used in all micro-level cutting machinery in industries.

Students can do many projects about wireless communication with the help of laser. We can communicate using laser beam transmission to the hills area in the case of mobile communication.