What is a laser?

LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser is a concentrated beam of light.

The light emitted from a laser is basically all the same wavelength or colour of light (monochromatic). When white light is split up, all the colours of the rainbow are visible. If a red laser is split, however, only red light is found.

Unlike light from other sources, such as that from a light bulb, laser light stays as a tight beam over large distances. However, like all light it does spread out a bit.

The waves of light that come from a laser are all lined up perfectly. This is called coherent light and is vital for many uses of lasers.

Lasers can be classified into two categoties; continuous and pulsed lasers. An example of a continuous laser is one that uses gases to generate a continuous beam. An example of a pulsed laser is one which uses either crystal, glass or a semiconductor as the medium to create a pulsating beam.

There are four classes that lasers can be grouped into depending on their power:

• Class 1 considered safe. This group includes lasers that can not emit levels of radiation above the exposure limits of the eye.
• Class 2 must emit a visible beam of light. Lasers in this group are too bright to stare into for long periods. Short viewing is not hazardous, however.
• Class 3 can emit any wavelength of light. It is not a fire or skin hazard. This group includes any continuous laser that is not class 1 or 2 and has an output power of less that half a Watt or less.
• Class 4 is any laser that has an output greater than half a Watt. These lasers are a fire and skin hazard and have very strict control measures put around them.

Uses for Lasers

Lasers have wide uses in today's life. They are found in devices such as CD-ROM drives or CD players to read data on CDs. They are also used in supermarket checkouts to interpret barcodes. Lasers also have many applications in medicine, such as in the removal of scarring or tattoos from a person's skin.

In optical networks, lasers are used to transmit information through optical fibres. This is accomplished by flashing or turning on and off the laser thousands of times a second. The binary system of numbers is normally used to transmit this information. The two states 0 and 1 are represented by either the presence or absence of the laser light.

Lasers can be used to measure everything from temperature to speed. Because a laser does not need delicate probes and because they can be extremely accurate, they are ideal for use in rugged applications and where a high level of accuracy is needed.

How Lasers Work

Lasers control how excited atoms release photons. A medium is stimulated to get its atoms into an excited state. This is normally done with intense flashes of light or electrical discharges that come from a device called a flash tube. After the medium has been stimulated it will have lots of atoms with electrons that have high levels of energy. These electrons get rid of this energy as photons (light energy).

The wavelength of the light that is released depends on the state of the electrons energy. Identical atoms with electrons in the same states will release photons of identical wavelengths.

A laser has a mirror at each end of the medium. Photons with a certain wavelength and phase travel back and forth reflecting off the mirrors. As they do this they stimulate other electrons in the medium to release some of their energy as photons of the same wavelength. Soon there are lots of photons all identical wavelength.

The mirror at one of the ends is a 99% reflective mirror. This means that it reflects most of the light for the propagation of more photons but lets some through. This light that passes through is the laser beam.

Ruby Laser Example

The following images depict how a simple ruby laser works. The ruby rod is the medium used to create the laser light.

Types of Lasers

Lasers are normally grouped according to the medium used to create the laser light.

• Solid state lasers use a solid laser creating medium. A ruby laser is an example of a solid state laser, using a tube of ruby to create the laser light.
• Semiconductor lasers, or diode lasers, are usually very small, use only a small amount of power and a very weak. Diode lasers emit a red beam of light between the wavelengths of 630 an 680nm. These lasers are not solid state lasers. They can be found in devices such as laser pointers and in optical networks.
• Dye lasers use organic dyes such as rhodamine 6G. The dyes are mixed or suspended in a liquid solution. This type of laser can output many different wavelengths of light.
• Gas lasers use gases as the laser creating medium. The most common types are helium and helium-neon gas lasers. Gas lasers usually output red light. Lasers using carbon dioxide output light in the far infrared wavelength range and are very powerful. They can be used to cut through hard substances such as steel. Infrared radiation is heat and so the laser is basically melting whatever it is pointing at.
• Excimer lasers (excited dimers) are lasers that use reactive gases mixed with nonreactive gases. When stimulated electrically a molecule is produced. This molecule produces ultraviolet light when it is stimulated in a laser. Reactive gases that can be used in a excimer laser are gases such as chlorine and fluorine, nonreactive gases that can be used include argon, krypton and xenon.

Although there are many different materials used for the laser creating medium, all lasers work using the same basic principals.