I. Gas Lasers
The gas laser is a laser in which the lasing medium is in the gaseous state. This kind of laser can be built to emit many different wavelengths in a wide variety of gases with output powers ranging from a milliwatt to over ten kilowatts, so a wide variety of experimentation is posible using gas lasers. Gas lasers can also emit light for almost any time length, ranging from sub-millisecond pulses to continuous-wave (cw) beams. These variables depend on what lasing medium the experimenter is using. Gas lasers are the most widely used variety of laser because they are cheap, flexible, and very easy to manufacture. They are the most common laser used in the classrooms of universities and high schools.
The basic function and process of a gas laser is similar to the function of the dye laser. The desired gas is contained in a tube, usually long, narrow (relative to its width), and of cylindrical cross-section. For the medium to become excited, an electric current is passed through the gas. When this happens, the liberated electrons transfer energy to atoms or molecules in the laser gas and the gas becomes excited. The excited atoms relax to a metastable state and subsequently undergo stimulated emission; the photons emitted resonate in the optical cavity, the same as dye laser, and emerge to form a laser beam.
The most common gas laser, the Helium-Neon ("He-Ne") is used for reading labels at supermarket check-out counters, recording holograms, and is the most common laser used for work in school laboratories. The Helium-Neon laser is a little different from single dye and single gas lasers in that there are two gases, as the lasing medium, which causes the laser to operate. The helium is excited by collisions of energetic electrons from the 'plasma' (mixture of ions, free electrons, and neutral atoms in a gaseous state) maintained between a pair of high-voltage electrodes. When an excited helium atom collides with a neon atom (outnumbered 5-to-1 by the helium), the excitation energy of the helium can be transferred to the neon since the energies of some of their excited states are similar (the 2s state of helium, for example, is close in energy to both the 4s and 5s states of neon). The energy levels of the neon atoms thus excited are metastable, so the electrons stay in these energy levels for a relatively long time. A chance de-excitation of one of the excited neon atoms will induce stimulated emission in others, and trapping that radiation in an optical cavity is then all that is needed to produce laser light.
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II. Semiconductor Lasers
Another type of laser is a semiconductor laser. Semiconductor lasers are commonly used in CD players and CD-ROMs. Semiconductor lasers typically emit in red wavelengths, although recent experiments have produced shorter wavelengths of orange, green, and even blue. However, it is still quite difficult to produce these wavelengths outside the laboratory for practical use.
Semiconductor lasers have a current flowing through differently-doped materials to create and emit light. A "semiconductor" is a type of material that has properties of both a conductor and an insulator. In the atomic structure of a conductor, the outer electrons move freely and can generate a current. In an insulator, these electrons in the outer energy levels cannot move enough to carry an electric current. A semiconductor has some, but not all, of its electrons moving in the outer energy levels. There are two types of energy bands in a semiconductor, a lower level valence band and an upper level conduction band. Most electrons are present in the valence band, and the conduction band (where the electrons can freely carry current, has relatively few.
To dope a semiconductor means to introduce in it either electron-donor or electron- acceptor 'impurity' atoms. The two different types of semiconductors are called n-type (if doped with donors) and p-type (acceptors). The n-type semiconductors usually start out as silicon or germanium. Silicon has four electrons in the outer shell. Crystals of silicon form in which each of the electrons bonds covalently between two atoms. Doping the silicon semiconductor with antimony or arsenic, for example (both with five valence electrons, as opposed to silicon's four), leaves the crystal with some 'free-agent' electrons, which are negatively charged current carriers, hence the name n-type semiconductor. The p-type semiconductor is similar except that the doping atoms have only three valence electrons. All three of the electrons form bonds with other atoms, but a 'hole' is formed in the crystal where the fourth electron would usually be. This hole is absence of the negatively charged electron, so p-type semiconductors get their conductivity from the presence of the positively charged holes.
The joining of both types of semiconductors creates a boundary zone between the p- type and n-type regions, known as a 'junction'. This is where the "action" occurs, including light emission. The amount of light emitted depends on this junction or gap. In a simple way of looking at a junction, surplus electrons from the n-type material can recombine with holes in the p-type material and emit light of the same energy as the difference in energy between the donor and acceptor energy levels. In the cavity of a semiconductor laser, known as the resonator cavity, there are small wafers of diode laser chips which are set perpendicular to reflect the light to transmit other light. This chip inside the resonator cavity oscillates and generates the amplification and laser beam.
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III. Solid-State Lasers
A Solid-State laser is a laser which contains a lasing medium of a crystal or glassy material, which is electrically non-conducting, and excited by light from an external device. The basic principles of Solid-State lasers are very similar to the principles of the tunable dye laser. The crystal is usually in the shape of a rod with mirrors at the ends, which forms a resonant tube within the rod when excited. When a flashlamp, arc lamp, or other laser is used to excite the rod, a population inversion is achieved in much the same manner as happens in gas and liquid lasers This laser is much less common than gas lasers because the procedure for growing the crystal and cutting it into a rod is a long and tedious process.
Ruby was the first crystal used to make a Solid-State laser - as well as being the first laser to be created at all! The drawback to the ruby is that it is limited to pulse operation, and only at low repetition rates. The ruby crystal itself is a synthetic crystal made in a laboratory by doping aluminum oxide, which crystallizes to form sapphire, with 0.01 to 0.5% chromium. The ruby laser is a three level system. At the ground state the chromium absorbs light in two bands, one at 550nm and the other at 400nm. The excited chromium atoms release energy as vibrations of the crystal and drop down at two closely spaced metastable levels. These levels last long enough to serve as the upper level of a 694.3nm laser transition to the ground state creating a red beam. This laser is rather inefficient and requires active cooling; either a water or air system.
All of these kinds of lasers basically operate in the same manner as the tunable dye laser. The only differences are each of the laser functions. Each of the four laser types have different kinds of lasers within each type, which serve many different purposes. The semiconductor laser has the biggest differences in the operations than all the other ones. This laser uses oscillating chip for reflection and amplification of light to create a laser beam. Most of the other lasers use an outside source to excite the lasing medium and mirrors to resonate the light to diverge into a laser beam. With a basic background of these other types of lasers we now can compare and analyze the other laser to the tunable dye laser to construct a more efficient dye laser.