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The term laser stands for light amplification by the stimulated emission of radiation, and what this means is that laser light is always monochromatic (of one specific wavelength) and is also spatially and temporally coherent.
Lasers are an essential component of scientific laboratories where they are used for chemical identification, fundamental physics research or the cauterization of biological tissue. They are not only limited to high-tech applications though, lasers also play essential roles in mass produced technologies from DVD players to laser pointers.
Lasers can have a very broad range of emission powers and frequencies depending on the application they are required for, and different types of laser require different lasing mediums to function.
This means that there are thousands of different types of lasers which are divided into different types according to the lasing medium they use. The most common ‘types’ of laser are gas, liquid, solid-state and semiconductor lasers.
Gas Lasers
Gas lasers use an electric current discharged through a gas medium to produce light. The most common gas laser is the helium-neon laser, but others include argon ion lasers, carbon dioxide and carbon monoxide lasers, nitrogen lasers and hydrogen lasers. The gas that is used determines the wavelength that the laser produces. Gas lasers are used where a high beam quality and long coherence length is needed, and as high powers are possible, they are frequently used for the cutting of hard materials.
Excimer lasers are a type of gas laser that uses a combination of both reactive and inert gases as a lasing medium. For example, chlorine or fluorine can be mixed with gases such as argon, xenon or krypton. The name is short for ‘excited dimer’, because they produce a pseudo dimer molecule when stimulated in the ultraviolet range. Excimer lasers are frequently used for semiconductor photolithography and LASIK eye surgery.
Liquid Lasers
The most common type of liquid lasers are dye lasers, which use organic dyes as the laser medium. Dye lasers generate light from excited energy states of organic dyes which have been dissolved in liquid solvents. Dyes can be used at a much broader wavelength than other types of laser due to the variety of dyes available and how easily tuned they can be.
Dyes that are often used include stilbene, coumarin and rhodamine 6G, but many more are available. Liquid lasers are used mainly for research and in laser medicine.
Solid-State Lasers
Solid-state lasers use solids such as glass or crystaline materials as laser mediums, with ions of rare earth elements introduced as impurities via doping. Common materials include sapphire, neodymium-doped yttrium aluminum garnet, neodymium-doped glass and ytterbium-doped glass.
In solid-state lasers, light energy is used as a pumping source. Using solid state lasers is a common method when removing tattoos.
Semiconductor Lasers
The final type of lasers are semi-conductor lasers, which are also known as laser diodes. In semiconductor lasers, the junction of the semiconductor diode is the laser medium. Semiconductor lasers also use an electrical pumping source, which stimulates the emission of light.
Semiconductor lasers are the lasers used frequently in consumer goods as they are cheap, compact, and use little power.
Categorizing Lasers by their Pulse Duration
Lasers can also be characterized by the duration of the laser emission, which can be continuous wave, pulsed, Q-Switch, repetitively pulsed or mode locked.
Continuous lasers operate with a stable average beam power that can be adjusted in high power systems and is fixed in low power gas lasers.
Single pulsed lasers are also known as long pulse or normal mode lasers. They have pulse durations of a few hundred microseconds to a few milliseconds. Single-pulsed Q-Switch lasers use a Q-switch cell to allows the laser media to store a maximum of potential energy before emission occurs in single pulses.
Repetitively pulsed lasers, also known as scanning lasers, use pulsed lasers at fixed or variable pulse rates that range from a few pulses per second up to as 20,000 pulses per second.
Mode locked lasers use resonant modes of the optical cavity to affect the characteristics of the output beam. When the phases of different frequency modes are locked together, the different modes interfere with each another to generate a beat effect, resulting in a laser output with regularly spaced pulsations.
Categorizing Lasers by their Safety Rating
A final way of classifying lasers is splitting them into different categories based on their hazards. The power, pulse duration and wavelength combined determines the classification for the rules and regulations of safety for laser users.
The different classifications are Class 1, Class 1M, Class 2, Class 2M, Class 3R, Class 3M and Class 4. Lasers can be visible, invisible or both, so precautions need to be taken for the higher classifications of lasers.
The safest laser classification is Class 1, which is eye-safe under all operating conditions. Class 1M is safe for viewing directly with the naked eye but is considered to be hazardous if viewed with the aid of optical instruments. Radiation in classes 1 and 1M can be visible, invisible or both. Both classes 1 and 1M are in enclosures that limits access to the laser radiation, for example CD Rom players.
Class 2 lasers are visible lasers that are considered safe for accidental viewing, but they should not be looked at deliberately for longer than 0.25 seconds. Like Class 1M, Class 2M is considered to be more hazardous if viewed with the aid of optical instruments. An example of a class 2 laser is a supermarket scanner.
Class 3R lasers are considered low risk but potentially hazardous. The output is between 1 and 5 times Class 1, and they are commonly used in laser pointers. Class 3B is considered to be dangerous compared to previous categories. Viewing of the diffuse reflection is safe, but they are hazardous to the eye or skin if the direct beam is viewed. For a continuous wave laser, the maximum output into the eye must not exceed 500mW.
Class 4 is the most dangerous classification, as they are capable of setting fire to materials as well as causing eye and skin injuries through direct and reflected exposure.
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