The word “LASER” is an acronym for Light Amplification by Stimulated Emission of Radiation. Simply stated the Laser is nothing more or less than a light amplifier. Laser amplifies Light by absorbing and emitting energy. The means of absorption can be either electronic on the atomic level or rotational / irrational on the molecular level. The emitted energy is at high intensity beam of Laser light. To understand the mechanics of amplification in ND: YAG lasers, we must look to the transitions that occur between energy levels of the atom (atomic level).

So how does the Laser amplify the light? At the heart of all lasers is the lasing medium, which contains atoms that can be stimulated to spontaneously emit light. The medium may be a gas mixture (Co2, helium-neon, etc.), a semiconductor substrate (Laser diodes), a liquid (dye lasers), or a solid crystal (Nd:YAG, Nd:YLF, Ruby, etc.). The laser will also have an energy source to excite (pump) the atoms of the media. The pump source will usually be either an electric discharge, light from a hi-intensity light source, or another Laser.         
                    Optical feedback is created by placing a mirror at each end of the media to reflect the photons traveling along the longitudinal axis back into the media. The reflected photons cause the upper level atoms to emit their photons by stimulated emission. As this process continues, one photon becomes two, two becomes four, four becomes sixteen, etc. – LIGHT AMPLIFICATION!! All of the photons will be the same wavelength, will be in phase and will travel the same direction along the path of reflection. The population inversion insures that there is always a sufficient quantity of atoms in the upper level state to sustain the Lasing process.

One of the mirrors of the optical feedback is coated to “leak” a small percentage (typically 10% or less) of the amplified light reflecting between the mirrors. This leakage is the laser output beam.

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