A. Laser
The term "laser" is an acronym for Light Amplification by Stimulated Emission of Radiation.

A laser beam is composed of focused coherent light, that is photons traveling on the same axis and at the same wavelength. A laser beam has tremendous energy because the light is tightly collimated; it does not diverge from its intended path. This creates a density of power, transferring maximum energy to the target. The fact that the light in the beam is all of one wavelength also helps increase total effective power on the target.

The laser beam is created in a lasing medium, a material whose atoms become excited and then decay, emitting photons to create the coherent beam.

An atom is excited when the electrons of the atom absorb excess energy above their natural state (called the "ground state"). When an atom decays, it releases the excess energy in the form of a photon. A photon is a 'particle' of light, which travels in the form of an electromagnetic wave .

To create a laser beam there must be a large number of excited atoms. When the large number of atoms are excited, the state is described as a population inversion. This does not occur naturally as atoms will tend to decay to their ground state if left alone. Thus, to create the population inversion, some pumping method is employed to impart extra energy to the atoms and keep them excited. This can be a high energy flashbulb, an electrical charge or even another laser.

When one atom decays naturally, its lost photon flies off in a straight line until it runs into something. If that something is another excited atom, it will usually stimulate decay in that atom, emitting more photons. As a law of nature, these photons will travel in the same path. If these hit more atoms, then more photons are emitted in that direction, in a cascading effect. This effect is called Stimulated Emissions.

A laser takes advantage of this one-axis cascade. Lasers are usually tubes, with mirrors at either end of the long axis of the tube. As the photons are created, some will travel along the long axis of the tube. These will hit the mirrors and be sent back along the long axis of the tube, creating more photons as they go . After several iterations of this process, a significant beam is created. Eventually, this beam will be strong enough to pass through one of the mirrors (a special mirror which is semi-transparent called a half-mirror) and exit the laser unit.

When the atoms in the lasing medium have decayed to their ground state, the pumping method will impart excess energy again, readying the atoms for more photon emissions.

B. Important Components of a Laser
The relevant parts of the laser are the lasing medium, the pumping method, and the optical cavity.

The lasing medium is the material containing the atoms which will become excited, then decay to create the beam. This can be a solid material (usually crystalline), a gas, or a liquid.

The pumping method is the process whereby excess energy is imparted to the atoms. The pumping method is typically an electron flow across the tube or a very bright light (photons) through the lasing medium. Diode lasers are sometimes used as the pumping method in other types of lasers.

The optical cavity is the enclosure that contains the medium. For example, if the laser is a CO2 gas laser, then some bottle must hold the gas. This bottle is usually a glass or metal tube with mirrors at either end. In the case of solid state lasers, the lasing medium is solid and is shaped into the form of a tube and essentially becomes its own bottle - the medium is the optical cavity!

C. Types of Lasers
There are several types of lasers in the world. These are generally classified by the lasing medium employed. For Exatron�s purposes there are three relevant types: Solid State (crystal), Gas, and Semiconductor . Each type of laser will produce a coherent beam of light at a set wavelength, although each type is a different wavelength.

1. Solid State (crystal) [Nd:YAG]
   Solid State lasers use a crystal of some material which has been "doped" with an impurity. The laser relies on the atoms of the impurity to become excited and then decay to form the coherent photon beam.

   The "YAG" laser is a Solid State (crystal) laser. YAG stands for the type of crystal involved: Yttrium Aluminum Garnet. The impurity in most YAG lasers is a rare element called Neodymium (Nd). Thus, the designation: Nd:YAG.

   The YAG crystal is both the lasing medium and the optical cavity. Either end of the cylindrical shaped crystal will be polished and finished in such a way as to create the mirrors discussed above.

   YAG lasers are excited by two different types of pumping methods:

   • Flashlamp
     The typical and traditional method is the use of a very high power flashbulb (the flashlamp). This flashlamp shines into the crystal, "pumping" photons into the crystal. These photons excite the atoms of the Neodymium and start the lasing process.
   • Diode
     The most recent development in YAG lasers is the use of a diode laser to excite the Nd:YAG crystal. These lasers have only come on the market in the past year and are still in their infancy.

   Nd:YAG lasers are typically high powered lasers used in cutting and machining operations. Exatron uses a low-power version YAG for marking applications.

   Nd:YAG lasers create beams at 1.064 micrometers (µm), which puts the light in the "near infrared" range of the electromagnetic spectrum. There is also a "frequency doubled" Nd:YAG at 0.532 µm in the green visible light spectrum.
   

2. Gas Lasers [CO2]
   Gas lasers employ some form of gas as the lasing medium. The gas is contained in a tube of glass or metal with mirrors at either end, creating the optical cavity. Gas lasers typically employ an electrical current as the pumping method. The current is applied to either end of the tube, and a current is arced through the tube. The flowing electrons of the current excite the atoms they hit as they travel through the tube.

   Neon lights and Fluorescent bulbs operate on this same principle. Shooting electricity through an inert gas, atoms of the gas are excited and decay into photons. These photons are visible because they are in the visible light spectrum and the optical cavity is transparent, letting them out.

   Gas lasers can be very high power, into the kilowatt range. Exatron's gas lasers are typically Synrad 10-watt sealed discharge tube CO2 lasers. These use an aluminum optical cavity and are pumped using the electricity flow described above.

   Gas lasers are used in many of the same applications as the solid state lasers: welding, cutting, marking, heating, etc.

   Note that there are different types of CO2, including sealed discharge tube, axial gas flow, and transverse gas flow. Exatron only uses a sealed discharge tube laser. The gas flow lasers are much larger, far more complicated (involving external supplies of specially-formulated gas), and used primarily for very-high energy applications such as metal cutting and welding.

   CO2 lasers create a beam at 10.600 µm, in the "far infrared" spectrum.
   

    

3. Semiconductor ("Diode") Lasers
   Semiconductor lasers are built around layers of silicon in the form of a semiconductor diode . The lasing medium is the silicon material. Semiconductor lasers use an electrical flow across the diode as the pumping method. The optical cavity is usually a small metal can in which the diode is encapsulated. Semiconductor lasers are very small as compared to the other forms of laser, but are very weak; being measured in milliwatts. Semiconductor lasers are used in many applications, including CD players, grocery store scanners, laser pointers, and as the pumping method in other lasers.

   Work is progressing among laser manufacturers on a new generation of diode lasers in the 10 watt range which may replace other forms of lasers altogether in the medium-power applications. These lasers are merely in the development stages and no prediction can be made as to availability.

   Semiconductor lasers create beams in several wavelengths, ranging from the visible spectrum to near infrared.

4. Other Lasers - Dye and Eximer
   There are two other types of lasers. Dye lasers incorporate an organic dye in a liquid solution. These are remarkable because they are tunable, that is the wavelength of the emitted light can be changed by adjusting certain operational parameters of the laser unit. Dye lasers are used almost exclusively in scientific applications.

   Eximer lasers are gas lasers, but are "reactive" in nature. Most gas lasers use an inert gas whose atoms gain and lose energy, but remain elementally intact (a CO2 molecule gains energy but remains a CO2 molecule). Eximer lasers incorporate multiple types of gasses which react and form new molecules when excited, and then decay into their original constituents when decaying. Eximer lasers are still largely used for research and have only recently found industrial applications. They are not used for marking applications.