Different Types of Laser

A solid state laser is one in which the atoms that emit light are fixed within a crystal or a glassy material.The first laser invented by Maiman in 1960 , the ruby laser, was a solid state laser. The atoms that emit light in solid state lasers are dispersed in a crystal or a piece of glass that contains many other elements. The crystal is shaped into a rod, with reflecting mirrors placed at each end. Light from an external source (such as a pulsed flash lamp, a bright continuous arc lamp, or another laser) enters the laser rod and excites the light-emitting atoms. The two mirrors form a resonant cavity and the inverted population in the laser rod, provided the feedback needed to generate a laser beam that emerges through the output mirror.

As the photons traverse the crystal, they stimulate the emission of additional photons until enough energy is available for a pulse of photons to break through the thinly mirrored end on the right of the laser crystal.

The Nd:YAG laser is a good example of the most commonly used solid state lasers. The laser medium is made up of yttrium- aluminium-garnet, with trivalent neodymium ions present as impurities. The laser transition involved corresponds to a wavelength of 1.06 mm, in the near infrared region (Fig.8,9).

A unique ,and perhaps the most important, type of laser in terms of opto- electronics applications is the semiconductor laser. It is unique because of its small dimensions (mm x mm x mm) , and its natural integration capabilities with micro electronic circuitry, Furthermore, the light amplification by the process of stimulated emission is not exactly in the form that we have described before.

A semiconductor laser uses special properties of the transition region at the junction of a p-type semiconductor in contact with an n-type semiconductor. In semiconductor materials, because of the extensive interaction of energy between atoms, the energy levels form bands. Energy band diagrams for an n-type and a p-type semiconductor are depicted in Fig.10. The energy gap between the valence band and the conduction band is designated by E_g and is measured in electronvolts,e.g., the Fermi level E_f is the level that divides the occupied from the unoccupied levels.

In a p-n junction, as shown in Fig.11, the energy levels readjust in accordance with thermodynamics so that the E_f band is the same through the junction. The valence band E_vv and the conduction band E_c of the p-type semiconductor are higher than the corresponding bands of the n-type semiconductor. If a positive voltage is applied on the p side (the so-called positive bias), the electrons on the n side will be attracted by the applied voltage and will cross into the junction region. There they recombine with the holes that have been pushed into the junction region by the positive bias. This process will continue as long as the external circuit is on, because the electrons and holes that have recombined are continuously replenished.

When the electrons and holes recombine, they emit energy in the form of photons. The junction transition region in which this takes place is therefore the source of radiation, and may be viewed as equivalent to the E₂ and E₁ transition levels which we discussed earlier.

To obtain stimulated emission and amplification from this region, the equivalent of the population inversion needs to be created, for which a high density of electrons and a high density of holes must exist simultaneously in the junction region. To achieve this, heavily doped p-n junctions are used in semiconductor lasers.

Fig.12a shows the resultant energy levels. When a positive bias is applied on the p-side, there is a transition region with a high concentration of electrons and holes, as shown in Fig.12b. This region serves as a population inverted medium, which amplifies the radiation emitted within it through electron - hole recombination. A p-n junction semiconductor laser is illustrated schematically in Fig.13. The shaded area is the transition region where the laser action takes place. This region is about 1-2 mm thick, and tens of micrometers long. As a result, the emission is squeezed into a thin plane, leading to an elliptical cross-section of the beam.

• Ion and Metal Vapour Laser:

  Operated at high temperatures to keep the metals (e.g., copper, gold etc.) vaporized and produced laser light in the infrared, visible or ultraviolet regions. They are excellent sources of short, high-intensity laser pulses at very high pulse-repetition rates.The mixture of metal vapour and noble gases are excited by electrical discharges. Example is copper vapour lasers which produces green and yellow light from a mixture of copper vapour with helium and neon.

• Carbon Dioxide Laser.

• The excimer Laser.

• The liquid (dye) Laser :

  Dye lasers use liquid organic dyes. Dye lasers can produce a broad and almost continuous range of colors, mainly in the visible part of the spectrum. With proper optical system any colour can be selected or tuning from one colour to another can be done. That is why dye lasers are particularly suited for applications in which a precise colour is required. Usually another laser source e.g., copper vapour lasers are used to excite the dye.

• The Free Electron laser.