LASER

LASERS--NO LONGER RESTRICTED TO CRIME-THRILLERS


A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons.
The term "laser" originated as an acronym for "Light Amplification by Stimulated Emission of Radiation".
The emitted laser light is notable for its high degree of spatial and temporal coherence.

CHARACTERISTICS OF LASER LIGHT:
Laser light is very different from normal light.

• The light released is monochromatic.
•  It contains one specific wavelength of light (one specific color).
•  The light released is coherent. It is “organized” -- each photon moves in step with the others.
•  The light is very directional. A laser light has a very tight beam and is very strong and concentrated.

BOHR'S ATOMIC THEORY:

• When an electron absorbs energy either from light (photons) or heat (phonons), it receives that incident quantum of energy. But transitions are only allowed in between discrete energy levels such as the two shown above. This leads to emission lines and absorption lines.

• When an electron is excited from a lower to a higher energy level, it will not stay that way forever. An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. 

When such an electron decays without external influence, emitting a photon, that is called "spontaneous emission".

• These excited state electrons can return to their resting state by emitting energy in the form of photons.The photon emitted has a very specific wavelength (color) that depends on the state of the electron's energy when the photon is released. Two identical atoms with electrons in identical states will release photons with identical wavelengths.

Working of a laser:
In order to produce the light rays associated with a laser thrre principles must be followed in the following order.

METASTABILITY:

STATE 1 IS METASTABLE  WHILE STATE 3 IS THE MOST STABLE

 Upon receiving certain quanta of energy ,electrons transit to another energy state (say E1 to E3).Since the stability of E3 is of the order 10ex-8 sec hence the electron soon decays to a lower energy level (say E2).However E2 is a metastable state since its life time is of the order 30ex-3 sec.

POPULATION INVERSION:

A number of problems limit the effectiveness of this approach. The central problem occurs because the lower laser level is the ground level, which is the normal state for most atoms or molecules. In order to produce the population inversion, a majority of ground state electrons must be promoted to the highly excited energy level, requiring a significant input of external energy. In addition, the population inversion is difficult to sustain for an appreciable time, and therefore, three-level lasers must be operated in pulsed mode rather than continuously.

As a result of metastability of E2 energy level,there are far more electrons in the E2 than in the E1 level.This is called population inversion as the rate of flow of electrons from E3 to E1 is greater than from E2 to E1.
This is achieved by very intense flashes of light or electrical discharges.

STIMULATED EMISSION:
Now that the electrons are in high energy levels the  stimulated emission must occur.In the process,an incoming photon stimulates an excited atom to give up its stored energy in the form of a photon that is identical in wavelength, direction, polarization, and phase to the stimulus photon.
If the excited atom is unable to produce a photon that matches the incoming photon, then stimulated emission cannot take place.
                                                                                         
As a photon passes through the collection of excited atoms, it can stimulate the generation of many trillions of photons, or more, creating an avalanche of light.

SUSTAINING STIMULATED EMISSION:
Two mirrors at either end of the lasing medium reflect these photons facilitating the light gain.The active medium can thus be regarded as an amplifier that takes in a small signal (one photon, say) and delivers a large signal (many photons, all identical to the first) at the output.

ASSEMBLY OF MIRRORS .ONE IS PARTIALLY AND THE OTHER FULLY SILVERED.

Monochromatic, single-phase, columnated light leaves the laser through the half-silvered mirror -- laser light!