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.

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

High Reflectance Mirror A mirror which reflects essentially 100% of the laser light.

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