The acronym LASER stands for Light Amplification by Stimulated Emissi

The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. A laser produces a beam of monochromatic light in which all the waves are in phase or are coherent.
Lasers contain four primary components regardless of style, size or application.

What is an Active Medium?
The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be excited to a metastable energy level by an energy source. 

What is an Excitation Mechanism? 
Excitation mechanisms pump energy into the active medium by one or more of three basic methods; optical, electrical or chemical. 

What is a High Reflectance Mirror?
A mirror which reflects essentially 100% of the laser light. 

What is a Partially Transmissive Mirror?
A mirror which reflects less than 100% of the laser light and transmits the remainder.

Lasing Action
Light is produced when energy is applied to an atom raising an electron to a higher unstable energy level. This electron will then randomly return to its stable energy level by releasing a photon of light. Light produced in this manner is called incoherent light and has many different wavelengths, directions and phases, depending on a number of factors.

When energy is applied to a laser active medium electrons are raised to an unstable energy level then spontaneously decay to a lower relatively long-lived metastable state. Electrons in this state will not spontaneously return to their ground energy level; therefore it is possible to pump large amounts of energy into the material thus obtaining a population inversion in which most of the atoms are in a metastable state. After this population inversion has been achieved, lasing action is initiated by an electron which spontaneously returns to its ground state producing a photon. If the photon released is of exactly the right wavelength it will stimulate an atom in a metastable state to emit a photon of the same wavelength (called Stimulated Emission). Many of these stimulated photons will be lost when they hit the side of the lasing medium. However if the photons travel parallel to the long axis of the laser optical cavity they will continue to stimulate emissions of photons having the same wavelengths which combine coherently until they reach the mirrored ends of the optical cavity. When the beam strikes the totally reflecting mirror in the optical cavity the beam is reversed and continues to stimulate emissions of photons which increase in intensity until the beam reaches the partially reflecting surface of the optical cavity. A small portion of the coherent light is released while the rest is reflected back through the lasing medium to continue the process of stimulating photons. Laser radiation will continue to be produced as long as energy is applied to the lasing medium.

What is a Continuous Wave Laser?
If energy is continuously pumped into the active medium an equilibrium may be achieved between the number of atoms raised to a metastable state and the number of photons emitted resulting in a continuous laser output. Output for continuous wave lasers is expressed as Irradiance(E) which is the concentration of laser power incident on a given area or Power/Unit Area (W/cm2).

What is a Pulsed Laser and a Q-Switched Laser?
Pulsed laser emissions are produced when the excitation medium is modulated producing a pulse of laser radiation lasting usually less than 0.25 sec. Pulsed output may also be produced by blocking the beam with a rotating mirror or prism.

Q-switching or Q-spoiling is a technique employed to produce a very high output pulse. Q- switching is accomplished by using a device to prevent the reflection of photons back and forth in the active medium. This produces a higher population of electrons in the metastable state. At a predetermined instant the Q-switch is turned off allowing the lasing action to continue producing very intense short pulses of laser radiation. Q-switched lasers produce pulses of 10 to 250 nanoseconds (ns).

Output for pulsed lasers expressed as Radiant Exposure (H) which is the concentration of laser energy on a given area Energy/Unit Area (J/cm2).

Wave Lengths Of The More Common Laser Types

Media	Wave Length (s)	Nanometers
Excimer Gas Lasers	Argon Fluoride	(UV)	193 nm
	Krypton Chloride	(UV)	222 nm
	Krypton Fluoride	(UV)	248 nm
	Xenon Chloride	(UV)	308 nm
	Xenon Fluoride	(UV)	351 nm