1. Very Useful link about LASER
http://www.colorado.edu/physics/2000/lasers/index.html

A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. The term laser is an acronym for light amplification by stimulated emission of radiation. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses.

http://en.wikipedia.org/wiki/Laser

In optics, stimulated emission is the process by which an electron, perturbed by a photon having the correct energy, may drop to a lower energy level resulting in the creation of another photon. The perturbing photon is seemingly unchanged in the process (cf. absorption), and the second photon is created with the same phase, frequency, polarization, and direction of travel as the original. If the resultant photons are reflected so that they traverse the same atoms or gain medium repeatedly, a cascade effect is produced. Stimulated emission is really a quantum mechanical phenomenon but it can be understood in terms of a "classical" field and a quantum mechanical atom. The process can be thought of as "optical amplification" and it forms the basis of both the laser and maser.

http://en.wikipedia.org/wiki/Stimulated_emission

The Pauli exclusion principle forces some electrons to be farther from the nucleus than others, which is why all the electrons in an atom do not simply occupy the 1s orbital. When electrons absorb energy either from light (photons) or from heat (phonons), they move farther away from the atomic nuclei but they are only allowed to absorb energy that will land them into specific energy levels. This leads to emission lines and absorption lines.

When an electron is excited, it will not stay that way forever. On average there is a half-life for any particular energy level after which half of the electrons initially in that state will have decayed into a lower state. When such a decay occurs, the energy difference between the level the electron was at and the new level must be released either as a photon or a phonon. When an electron decays due to "timeout" it is said to be due to "spontaneous emission." The phase associated with the photon that is emitted is random and has to do with some quantum mechanical ideas concerning the atom's internal state. If a bunch of electrons were put into an excited state somehow and then left to relax, the resulting radiation would be very spectrally limited (only one wavelength of light would be present) but the individual photons would not be in phase with one another. This is also called fluorescence.

Other photons (i.e. an external electromagnetic field) will affect an atom's state. The quantum mechanical variables mentioned above are changed. Specifically the atom will act like a small electric dipole which will oscillate with the external field. One of the consequences of this oscillation is it encourages electrons to decay to the lower energy state. When it does this due to the presence of other photons, the released photon is in phase with the other photons and in the same direction as the other photons. This is known as stimulated emission.

Stimulated emission can be modelled mathematically by considering an atom which may be in one of two electronic energy states, the ground state (1) and the excited state (2), with energies E1 and E2 respectively.

If the atom is in the excited state, it may decay into the ground state by the process of spontaneous emission, releasing the difference in energies between the two states as a photon. The photon will have frequency ν and energy hν, given by:

E2 − E1 = hν,
where h is Planck's constant.

Alternatively, if the excited-state atom is perturbed by the electric field of a photon with frequency ν, it may release a second photon of the same frequency, in phase with the first photon. The atom will again decay into the ground state. This process is known as stimulated emission.

The laser is aimed at a rotating polygonal mirror, which directs the laser beam through a system of lenses and mirrors onto the photoreceptor. The beam sweeps across the photoreceptor at an angle to make the sweep straight across the page; the cylinder continues to rotate during the sweep and the angle of sweep compensates for this motion. The stream of rasterized data held in memory turns the laser on and off to form the dots on the cylinder. (Some printers switch an array of light emitting diodes spanning the width of the page, but these devices are not "Laser Printers".) Lasers are used because they generate a narrow beam over great distances. The laser beam neutralizes (or reverses) the charge on the white parts of the image, leaving a static electric negative image on the photoreceptor surface to lift the toner particles.

http://upload.wikimedia.org/wikipedia/commons/thumb/1/1a/Laser_printer-Writing.svg/300px-Laser_printer-Writing.svg.png