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     Dr.(Phys.)Dipl.-Ing.Ralf-Udo Hartmann

Laser

What you should know about LASER!                                                                                                                
 
Laser has become such a common word that hardly anybody is aware of the fact that it is an abbreviation which means : "Light Amplification by the Stimulated Emission of Radiation". 
 
The laser is a light source emitting a densely bundled beam. The light beam has in contrast to the conventional light a certain wave length and its uniform waves are emitted on a parallel basis. There is a great variety of lasers. Their wave length range from infrared to ultraviolet colors and their output power varies from fractions of a milliwatt to over one gigawatt.
 
A laser lighting display or laser light show involves the use of laser light to entertain an audience. A laser light show may consist only of projected laser beams set to music, or may accompany another form of entertainment, typically a dance concert or other musical performance.
 
Laser light is useful in entertainment because the coherent nature of laser light allows a narrow beam to be produced, which allows the use of optical scanning to draw patterns or images on walls, ceilings or other surfaces including theatrical smoke and fog without refocusing for the differences in distance, as is common with video projection. This inherently more focused beam is also extremely visible, and is often used as an effect. Sometimes the beams are "bounced" to different positions with mirrors to create laser sculptures.
 
Laser scanners consist of small mirrors which are mounted on galvanometers to which a control voltage is applied. The beam is deflected a certain amount which correlates to the amount of voltage applied to the galvanometer scanner. Two galvanometer scanners can enable X-Y control voltages to aim the beam to any point on a square or rectangular raster. This enables the laser lighting designer to create patterns such as Lissajous figures (such as are often displayed on oscilloscopes); other methods of creating images through the use of galvanometer scanners and X-Y control voltages can generate letters, shapes, and even complicated and intricate images. (The use of X-Y raster scanning to create images is also used in television picture tubes.) A planar or conical moving beam aimed through atmospheric smoke or fog can display a plane or cone of light known as a "laser tunnel" effect.

1. Laser Light
 
In contrast to the spontaneous emission typical of the conventional light the laser beam is created by a stimulated emission. The photons leaving in the same direction rebound between two mirrors - the so-called resonator - creating a constant light wave in the laseractive substance. One of the two mirrors is semitranslucent in its center. Passing through the vertically hitting photons leave the resonator as parallel laser light having the same wave length. Photons which hit the side of the tube are emitted as heat. 
 
 
Spontaneous emission
 
When a photon is added to an atom by means of external influences (energy supply) an electron is put into a higher orbit. Because of the force of attraction of the atomic nucleus, however, it returns to its previous state within seconds and the released energy is emitted as light flash (photon). This happens in general spontaneously, not predictable and in any possible direction. 
 
Stimulated emission
 
In contrast to the spontaneous emission the emission of photons is caused on purpose. The atoms being already in a state of excitation are forced to emit their energy by adding another photon. The photons leave the atom in the same direction as the entering one. This effect appears only provided that the majority of the atoms of the laseractive substance are on a higher level of energy (inversion).
Stimulierte Emission
 
Inversion
 
Inversion means that a great number of the atoms of a substance are in a state of excitation. The laseractive substance has to be supplied with energy by means of the following possibilities:
 
1. Creation of the inversion by means of impact electrons
 
One possibility to supply a system with energy is ?????? This way the use of this procedure is restricted to one system: gas systems. This pump methods is typical of the gas lasers.
 
2. Creation of the inversion by optical pumping
 
As far as solids and liquids are concerned it is not possible to supply the necessary energy by impact electrons. The energy has to be supplied as light. By means of strong light sources (flash light or other lasers) the laseractive substance is put onto a higher level of energy and starts to emit laser light.

Bohr's nuclear model
 
An atom consists of an atomic nucleus containing positive protons and neutral neutrons. Negative electrons - consisting of the so-called photon or light quantum - revolve around this nucleus on different orbits. Bohrsches Atommodell
 
 
2. Laser Technology  ( Types of laser )
 
Besides several other characteristics as e. g. output power or wave length, the different laser types are first of all classified by the physical state of the laseractive substance. These groups are divided again into different sub-groups according to different criteria.
 
2.1 Solid state lasers
 
The most common lasers are ruby lasers, (Neodym integrated in Yttrium-Aluminium-Garnet) and Nd-glass lasers (Neodym installed integrated in Yttrium-aluminium glass). These types of lasers emit red and infrared beams. A very high output power can be achieved in the pulse operation thanks to the high amplification by the material. For this reason solid state lasers are used when impulses of a high energy are sought for example: Biochemistry and biophysics, not short-term-physics, medical studies and ophtalmology. 
 
2.2 Atomic lasers
 
This sub group of gas lasers did not penetrate into the market because the other types of lasers function much more reliable. Atomic lasers take advantage of the transmission of electrons between two laser levels of a neutral atom. Suitable elements are He, Ne, Ar, Kr, Xe, H, C, Si, Sn, Pb, N, P, O, S, F, Cl, Br and J. The use of nobel gas/innert gas is in general preferred.
 
2.3 Ion lasers
 
Ion lasers belong to the group of gas lasers. Their function is based on the laser levels of single-ionized or multiple-ionized elements. As some elements are able to laser both in their elementary state and after the splitting off of electrons, elements are used in this group which can also be found in the group atomic lasers. Metals have to be put into a state of steam by means of a heating system. Nobel gas (Ar, Kr or Xe) or the metal cadmium are preferably used. The nobel gas ion laser is the standard laser type for a visible and continuous emission of beams with higher mains. This type is used first of all for medical studies and ophtalmology, for the production of video disks and audio disks and also for holographics. Metal steam ion lasers are used in the field of information and measurement technologies.
 
2.4 Molecular lasers
 
Molecular laser differ from other types of lasers as far as the laseractive substance is concerned. The laser level is formed due to the different forms of vibration and rotation of the molecules but not by means of the electrons which emit light when they change the energy levels. The same is valid for excimer lasers - their molecules which do not dispose of a stable elementary state (so-called Excimers) emit energy during their disintegration. As molecular lasers may reach a very high output power during a constant use they are hardly ever used in pulsed operation. First of all the carbon dioxide lasers which emit infrared light are used to work on raw materials. The helium neon laser having a weaker output power is used in the field of holography, ordnance survey, leveling and light barriers.
 
2.5 Liquid lasers
 
For liquid lasers two groups of active substances are used: Lantanoids as for example europium, samaarium and terbium because dissolved in alcohol they show similar characteristics as the crystals in solid state lasers. It is not necessary to create crystals so that longer and stronger systems can be built. However, in spite of this advantage this group of lasers has not penetrated into the market. 
Frequently used is the group of pigment or dye lasers. Pigments as for example rhodamin, Nile blue, cresyl violet and cyanin are dissolved in alcohol or water. Nowadays more than 40 substances are used. Each one is suitable for another laser frequency, pumping frequency or purpose. Pigment lasers emit in general lines larger than 10 nm. They can easily be adjusted by means of elements like e. g. prisms, etc. Therefore it they are often used for research and analysis purposes. Depending on the pigment wave lengths from 300 nm up to 1000 nm are possible and in pulse operation the output power is up to 40 W.
 
2.6 Semiconductor lasers
 
Diode or semiconductor lasers, which only differ slightly from the usual light diodes as far as the size and the handling are concerned, are sold as compact components. Only the cooling is a little more difficult. The most important diode laser is the Galliumarsenid-Laser (GaAS). Semiconductor lasers are used in constant and pulsed operation. First of all the fast diode lasers having small and medium output powers are used for the digital data transmission. Because of their size, however, they are also used for consumer goods (laser printer, CD player etc.). Powerful laser diodes can be used for light barriers of several hundred meters and for optical switches. 
 
Operation characteristics of solid state lasers
 
The firsts lasers which were built were solid state lasers (Ruby lasers in 1960). Different solid matters made of crystal and glass were used. The laseractive substance is put into a state of excitation by means of optical pumping. There are resonator mirrors at both ends of the solid state rod. Both the pump light and the rod are surrounded by an ellipse-shaped reflector mirror so that as much light as possible hits the laseractive substance. Solid state lasers are first of all used in pulse operation.

 
Operation characteristics of gas lasers
 
The laseractive substance consists of gas or gas mixtures. The gas is inside a tube (discharge tube). This tube is hermetically closed at both ends with brewster windows. The two resonator mirrors are located on the outside of the discharge tube. The emission is launched by adding electricity. Gas lasers which are used in continuous or pulsed operation may have a very high output power, however, in usually they have to be cooled because of the high emission of heat. There are three different types of gas lasers: Atomic lasers, ion lasers and molecular lasers.
 
 
Operation characteristics of liquid lasers
 
Liquids used as laseractive substance are capsuled in a tube which is closed on both sides with resonator mirrors. By means of a pump light (optical pumping) the medium is activated. The most common liquid lasers are pigment lasers. Pigments dissolved in liquids or organic elements as laseractive laseractive substance allow the optimal adjustment of the laser. In addition to this they can be used in continuous or pulsed operation.
 
Way of function of a semiconductor lasers
 
Nowadays semiconductor lasers are the most common and smallest lasers. Regarding the semiconductor laser it is taken advantage of the fact that the atomic grid emits electrons. So called semiconductors dispose of more electrons (-), p-semiconductors of more Defektelektronen??? (+). If the voltage on both ends of a p-n semiconductor is reinforced electrons are transmitted from the p-area into the n-area. Light is emitted in the transmission area. The continuously operated semiconductors can be controlled precisely, however, the light quality is reduced.

 
3. Laser Technology  (Safety regulations ) 
 
The extraordinary energy which - the most interesting characteristic of the laser - might also become dangerous for human beings. Laser with even only a low output power may hurt the human eye if they are not handled correctly. Lasers having a high output power may cause serious burns or cuts. The danger depends both on the output power and on the wave length. The safety regulations are stipulated in standard specifications lists. Furthermore laser systems have to be checked after their installations in Germany by the German TÜV. Lasers having a high output power can only be operated by trained persons who are obliged to check if all safety regulations are observed. There are different types of classes according to the output power and wave length. All laser or devices containing lasers have to be marked with special danger signs.
 
 
Classes of lasers
 
There are four different classes of lasers according to the output power specified by the legislator. For lasers of class 3 and 4 key switches have to be used. 
 
Class 1
 
Lasers of this class do not cause any harm no matter how they are used, how long the person is exposed to the radiation or which wave length the laser has. Continuous wave (CW) emitting visible light and having a power of less than 0,0004 mW observe this regulation. 

Class 2

The CW-power of lasers of class 2 ranges between 400 nm and 700 nm

Class 3a

Lasers emitting a light between 400 and 700 nm and having an output power of less than 75 mW.

Class 3b

The CW-power is limited to 0,5 W.

Class 4

Devices having a high output power.


Danger signs

Danger
 
Lasers and the projection area of the laser has to be marked with danger signs. Additionally the manufacturer often indicates the class of the laser. Moreover further international danger signs are fixed on almost all lasers. 

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