The essential components of a laser are an external energy source, an amplifier medium, and a resonator. When a medium, for example an Nd:YAG crystal is arranged so between two mirrors that photons always trigger an “induced emission”, this is called an optical resonator. The supply of external energy also is called pumping. In this case, the energy is supplied by the intensive radiation of a thermal high-capacity light source, for example a flash lamp.
Not only solid materials (Nd:YAG, ruby crystals, or semiconductors), but also gases (carbon dioxide, nitrogen, helium-neon mixed metal vapors) or liquids (solutions of organic pigment molecules) are used as laser media. The wavelength of the laser radiation is determined by their special energy level.
The laser medium is located in a resonator, i. e. it is enclosed between two flat or curved mirrors. The photons travel back and forth between the mirrors, causing continuous light emissions in the medium, thus amplifying the generated beam. This realizes the formation of a standing wave, which is created when a wave is reflected and overlaps itself. This creates the impression that the wave has stopped. One of the two mirrors has a reflection factor of 100 %, the other one has a reflection factor of ca. 98 %, so that the generated standing wave can exit in part.
The strength of the exiting laser beam can vary from a few milliwatt to several megawatt.
As a rule, laser beams are divided into two types: “Pulsed lasers” and “continuous wave lasers”. While the continuous wave laser emits a continuous beam, the pulsed laser emits light energy in the form of light flashes not longer than a few milliseconds.