When lasers were invented in 1960, they were called "a solution looking for a problem. " Nowadays, lasers are ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military.
Having examined stimulated emission and optical amplification process in the "Lasers, Applications of Quantum Mechanics" section, this atom looks at how lasers are built.
A laser consists of a gain medium, a mechanism to supply energy to it, and something to provide optical feedback (usually an optical cavity). When a gain medium is placed in an optical cavity, a laser can then produce a coherent beam of photons.
The gain medium is where the optical amplification process occurs. It is excited by an external source of energy into an excited state (called "population inversion"), ready to be fired when a photon with the right frequency enters the medium. In most lasers, this medium consists of a population of atoms which have been excited by an outside light source or an electrical field which supplies energy for atoms to absorb in order to be transformed into excited states. There are many types of lasers depending on the gain media and mode of operation . Gas and semiconductors are commonly used gain media.
Wavelengths of Commercially Available Lasers
Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The height of the lines and bars gives an indication of the maximal power/pulse energy commercially available, while the color codifies the type of laser material.
The most common type of laser uses feedback from an optical cavity--a pair of highly reflective mirrors on either end of the gain medium. A single photon can bounce back and forth between the mirrors many times, passing through the gain medium and being amplified each time. Typically one of the two mirrors, the output coupler, is partially transparent. Some of the light escapes through this mirror, producing a laser beam that is visible to the naked eye.