There are a wide variety of lasers, covering a spectral range from the soft X-ray to the far infrared, delivering output powers from microwatts to terawatts, operating from continuous wave to femtosecond pulses, and having spectral linewidths from just a few hertz to many terahertz. The gain media utilized include plasma, free electrons, ions, atoms, molecules, gases, liquids, solids, and so on. The sizes range from microscopic, of the order of 10 μm3, to gigantic, of an entire building, to stellar, of astronomical dimensions. An optical gain medium can amplify an optical field through stimulated emission. If the gain medium is sufficiently long, it is possible to generate laser light at one end of the medium through amplification of some initial optical field from spontaneous emission produced at the other end of the gain medium. Astrophysical laser action in space has been found to occur naturally, for example at the deep ultraviolet wavelength of 250 nm from the star Eta Carinae, at the near-infrared H2 wavelength of 2.286 μm from the star NGC 7072, at the far-infrared wavelength of 169 μm in a disk of hydrogen gas surrounding the star MWC349 in the constellation Cygnus, and at the mid-infrared CO2 wavelength of 10.6 μmin the Martian atmosphere. In a practical laser device, however, it is generally necessary to have certain positive optical feedback in addition to optical amplification provided by a gain medium. This requirement can be met by placing the gain medium in an optical resonator. The optical resonator provides selective feedback to the amplified optical field.

Lasers are indeed fascinating, but not all of them are of practical usefulness as photonic devices.