Lasers contain four primary components regardless of style, size or application.

1. 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.

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

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

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