Laser Idea

Laser Idea:
[align=left]Let N1, N2 represent number of atoms of level 1, 2 in the atomic system, if a plane wave of intensity corresponds to photon flux F passes with the direction of Z-axis in a certain material (see fig. 1). So the change in this flux as a result of absorption or stimulated emission will be given by:
dF =σ F (N1 –N2) dZ ……………………………………………………………….(1)
from equation (1) we can notice that the material behaves as an amplifier if N2>N1, i.e. dF/dZ > 0 while it behaves as an absorber if dF/dZ < 0.
The number of atoms in levels 1, 2 in thermal equilibrium given by Boltzmann statistics;
(http://www.iraqup.com/uploads/I7upE-0Sh03943.JPG………………………………………………………………..(2)
Where:[/align]
[img][align=left]http://www.iraqup.com/uploads/QMEv5-yw8456952.JPG[/align][/img] Is [align=left]number of atoms in thermal equilibrium,
K: is Boltzmann constant.
T: is the absolute temperature[/align] [/align]

[align=center]http://www.iraqup.com/uploads/pFqJ5-0sV085130.JPG[/align]
[align=center]Fig. 1: change in photon flux when it passes through thickness dZ of the material.[/align]
[align=left]In ordinary conditions, we have N2 < N1 ,so the material behaves as an absorber at frequency γ .
If we assume that thermal equilibrium is not satisfied, so N2 > N1 and the material will behave as an amplifier, in this case a population inversion will take place in the medium (material ), and it will called "Active Material". To make an oscillator from an amplifier, we need positive feedback suitable for that, and in the case of laser, we can get the feedback by introducing the active material between two high reflection mirrors (for example plane- parallel mirrors; see fig. 2). In this case the electromagnetic wave will travel vertically to the mirrors and reflect forward and back between the mirrors, then it will be amplified by each pass through the active material, if we make one of the mirrors of partially transmittance we get an output laser beam. [/align]


[align=center]http://www.iraqup.com/uploads/a2K06-EEix87552.jpg[/align]
[align=center]Fig.2 :the active material between pair of mirrors.[/align]

[align=left]Then from that it can be found there are four functional elements are necessary in lasers to produce coherent light by stimulated emission of radiation. Figure 3 illustrates these four functional elements. [/align]


[align=center]http://www.iraqup.com/uploads/xEt7F-q3A741687.JPG[/align]
[align=center]Fig. 3: Elements of a laser.[/align]

[align=left] 1.ACTIVE MEDIUM[/align]

[align=left]The active medium is a collection of atoms or molecules that can be excited to a state of inverted population; that is, where more atoms or molecules are in an excited state than in some lower energy state. The two states chosen for the lasing transition must possess certain characteristics. First, atoms must remain in the upper lasing level for a relatively long time to provide more emitted photons by stimulated emission than by spontaneous emission. Second, there must be an effective method of "pumping" atoms from the highly-populated ground state into the upper lasing state in order to increase the population of the higher energy level over the population in the lower energy level. An increase in population of the lower energy level to a number above that in the high energy level will negate the population inversion and thereby prevent the amplifications of emitted light by stimulated emission. In other words, as atoms move from the upper energy level to the lower energy level, more photons will be lost by spontaneous emission—giving off randomly directed, out-of-phase light—than gained due to the process of stimulated emission.
The active medium of a laser can be thought of as an optical amplifier. A beam of coherent light entering one end of the active medium is amplified through stimulated emission until a coherent beam of increased intensity leaves the other end of the active medium. Thus, the active medium provides optical gain in the laser. And the active medium may be a gas, a liquid, a solid material, or a junction between two slabs of semiconductor materials. [/align]

[align=left]2. EXCITATION MECHANISM [/align]
[align=left]The excitation mechanism is a source of energy that excites, or "pumps," the atoms in the active medium from a lower to a higher energy state in order to create a population inversion. In gas lasers and semiconductor lasers, the excitation mechanism usually consists of an electrical-current flow through the active medium. Solid and liquid lasers most often employ optical pumps.[/align]


[align=left]3. FEEDBACK MECHANISM[/align]
[align=left]
The feedback mechanism returns a portion of the coherent light originally produced in the
active medium back to the active medium for further amplification by stimulated emission. The amount of coherent light produced by stimulated emission depends upon both the degree of population inversion and the strength of the stimulating signal. The feedback mechanism usually consists of two mirrors--one at each end of the active medium--aligned in such a manner that they reflect the coherent light back and forth through the active medium. [/align]

[align=left]4. OUTPUT COUPLER[/align]

[align=left]The output coupler allows a portion of the laser light contained between the two mirrors to leave the laser in the form of a beam. One of the mirrors of the feedback mechanism allows some light to be transmitted through it at the laser wavelength. The fraction of the coherent light allowed to escape varies greatly from one laser to another[/align].