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EN&LmЅo0ab>$&LBXP0 mbA$&LBXP||Є9lNݩ&Mȷ,cfNOr8[@|8*` g=[s fXC*وĠ7hو!CK8y_A:3IDATh,&L0a„ &L0a„ &L0a„ &L0a„ &L0Q"AtIENDB`!#( / 00DTimes New RomanTT ܖ 0ܖDArialNew RomanTT ܖ 0ܖ" DGaramond RomanTT ܖ 0ܖ0DWingdingsRomanTT ܖ 0ܖ C0. @n?" dd@  @@`` xXB   /Xb$s;",gu b$Ƃ~^![Nv  0AA@8 8ʚ;ʚ;g4LdLdP  02ppp@ <4dddd w 0T <___PPT10 pp6___PPT9|t(?  %O  =@Light Amplification by Stimulated Emission of Radiation (LASER)AA@   Is a mechanism for emitting light within the electromagnetic radiation region of the spectrum, via the process of stimulated emission. Properties of laser light: The laser light is (usually) narrow low-divergence beam, that can be manipulated with lenses. coherent light waves of identical frequency and phase. the laser light is a narrow-wavelength electromagnetic spectrum monochromatic light.  Z Z Z " Z Z$h  ^  6  T    Basic Principles of Lasers To explain the process of light amplification in a laser requires an understanding of the energy transition phenomena in the atoms of its active medium. They include: spontaneous emission, stimulated emission/absorption and non-radiative decay. The theory of quantum mechanics states that the electrons of atoms can take different energy states, E1, E2, E3, for example, with E1WU X   WStimulated Absorption If the atom is initially in the ground level E1, the atom will remain in this level until it gets excited. When an EM wave of frequency n0 is incident on the material, there is a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called Stimulated Absorption. Non-Radiative Decay Note that the energy difference between the two levels can decay by non-radiative decay. The energy difference can change into kinetic energy or internal energy through collisions with surrounding atoms, molecules or walls. ^X Z@0 X Population Inversion   Normally the population of the lower energy levels is larger than that of the higher levels. The processes of stimulated radiation/absorption and spontaneous emission are going on in the same time, yet even if we ignore the decay factors, stimulated absorption still dominates over stimulated radiation. This means that the incident EM wave cannot be amplified in this case. Amplification of incident wave is only possible when the population of the upper level is greater than that of the lower level. This case is called Population Inversion. This is a mechanism by which we can add more atoms to the metastable level and hold them there long enough for them to store energy, thereby allowing the production of great numbers of stimulated photons. Z PP&e       To do this, we pump atoms into the metastable level at a rate that exceeds the rate at which they leave. A large number of atoms are therefore excited to and held in this level, leaving an almost empty level below it. The atoms stay in this metastable level without de-exciting while the population builds up, giving rise to a population inversion. In practice laser action cannot be achieved for only two levels, as described above. Three and four level systems work however. An analysis of these systems follows, followed by a description of the pumping schemes for each system. (Note: A metastable level is one that has a long lifetime and the for which the probability of spontaneous emission is low. This favors conditions for stimulated emission. If an atom is excited to a metastable state it can remain there long enough for a photon of the correct frequency to arrive. 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