A laser is a device that emits light through a process of optimal amplification based on the simulated emission of electromagnetic radiation. The term “Laser” comes from the acronym for ” Light Amplification by Simulated Emission of Radiation. Laser differs from other sources of light because it emits light which is coherent. This coherence enables the light to be focused on a spot which enables applications such as laser cutting, lithography, laser pointers etc.
A laser consists of a gain medium, which is a mechanism to energize it and another to provide optical feedback. The gain medium is a material with properties that allow it to amplify light by way of simulated emission. Light of a specific wavelength that passes through the gain medium is amplified, ie increased in power.
For a gain medium to amplify light, it must be supplied with energy in a process known as pumping. The energy is typically supplied as an electric current or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser. The most common type of laser uses feedback from an optical cavity which are a pair of mirrors on either end of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and being amplified each time.
TYPES OF LASERS
- Gas Lasers
- Chemical Lasers
- Excimer Lasers
- Solid State Lasers
- Fiber Lasers
- Photonic Crystal Lasers
- Semiconductor Lasers
- Dye Lasers
- Free Electron Lasers
- Exotic Media.
Gas lasers are built using gases such as Helium and Neon. The helium- neon gas laser (HeNe) is able to operate at a vast number of different wavelengths but most are engineered to lase at 633nm. These types of lasers are commonly used in optical research and educational laboratories.
These lasers are powered by a chemical reaction allowing large amount of energy to be released quickly. These very high powered lasers are used by the military and for industrial purposes. Some examples include hydrogen- fluoride laser (2700-2900 nm) and deuterium – fluoride laser (3800 nm). The reaction is a combination of hydrogen gas or deuterium gas with combustion products of ethylene in nitrogen trifluoride.
These are special types of gas lasers powered by an electric discharge. The lasing medium is an excimer which consist of two atoms or molecules that would not bond if both were in the ground state, which means they can only exist when one atom is in an excited electronic state. Once the molecule transfers its excitation energy to a photon its atoms are no longer bound to each other and the molecule disintegrates. Excimers currently used are all noble gas compounds. Noble gases are chemically inert and can only form compounds while in an excited state. Excimer lasers usually operate at ultraviolet wavelengths and are used in semiconductor lithography and eye surgery.
Solid State Lasers
Solid state lasers use a crystalline or glass rod which is doped with ions that provide the required energy states. The first working laser was a ruby laser made from ruby ( chromium doped corundum). The population inversion is maintained in the dopant. These materials are pumped optically using a shorter wavelength than the lasing wavelength, from a flashtube or from another laser. Common dopants used for solid state lasers include, Neodymium, Ytterbium, Holmium, thulium, erbium and titanium.
Solid state lasers where the light is guided due to the total internal reflection in a single mode optical fiber are called fiber lasers. Fibers have a high surface to volume ratio which allows efficient cooling. The fibers waveguiding properties tend to reduce distortion to the beam. Erbium and ytterbium ions are examples of dopants used for such lasers.
Photonic Crystal Lasers
Photonic crystal lasers are lasers based on nano structures that provide the mode confinement and the density of optical states or DOS structure required for the feedback to take place.
These laser types are diodes which are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflections from the end of a crystal form an optical resonator which may be internal or external based on manufacturer design. These lasers especially the medium powered are used in laser pointers, CD/ DVD players and laser printers. The highest powered laser diodes with power up to 20 kW are used for industrial applications such as cutting and welding.
Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes or mixture of dyes allows these lasers to be highly turnable or to produce very short duration pulses.
Free Electron Lasers
These lasers generate coherent, high powered radiation that is turnable currently ranging in wavelength from microwaves to tetrahertz radiation, infrared to the visible spectrum to soft X- rays. They have the widest frequency range of any laser type. Free electron Lasers share similar characteristics with other lasers such as coherent radiation but their operation is quite different. Unlike gas, liquid or solid state lasers which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasting medium, hence the term free electron.
These are high powered lasers which are largely still in research and development stage. The concept is of a high quantum energy laser using transitions between isomeric states of an atomic nucleus. Some of the early studies were directed towards short pulses of neutrons exciting the upper isomer state in a solid so that the gamma ray transition could benefit from the line narrowing of the Mossbauer effect. While many scientists remain optimistic about a break through, an operational gamma ray laser is yet to be realized. The research in this area is still very much a work in progress.