Fundamentals of Laser Physics

Laser medium and illustration of active molecules

A laser is a source of coherent light. It contains an optical oscillator that increases the amplitude of an optical field while maintaining its phase. This coherent amplification is achieved through Light Amplification by Stimulated Emission of Radiation (LASER). The process of amplification is the result of stimulating an atom such that it emits an identical photon to one already present. This "clone" photon has the same phase, frequency, direction, and polarization as the original photon, meaning they are coherent. It is this coherent amplification of light that gives lasers their unique output characteristics.

The three key components of a laser are a gain medium, a pump source, and a resonator. The pump source is the mechanism by which a population inversion (necessary for generating stimulated emission) is produced in the gain medium. The gain medium is chosen to achieve efficient lasing operation as well the desired characteristics for the laser output. The resonator mirrors allow for selective reflection of incoherent photons produced by spontaneous emission back into the gain medium. Stimulated emission replicates these reflected photons and this sequential reflection/amplification process gives rise to a large number of coherent photons. Finally, a portion of these photons are transmitted through a partially reflective mirror, delivering the output laser beam. Newport | Laser Physics

Elasic collisions and kinetic energy transfer

Elastic collisions acting part of the processes that happen in lasre physics

A physicsl example of laser light production is a CO2 laser which is a gas discharge laser that gains its excitation from kinetic energy transfers from elastic and inelastic collisions between a large current of electrons and gas molecules. Unlike the others, however, it is a molecular laser, that is, it does not work by means of electronic transitions of atoms, but rather by means of kinetic excitations of the gas molecules. The lasing transitions take place between phonon vibrational states of the molecule. Optical Materials (Second Edition) 2021, Pages 369-428