# Is a purely classical description of lasers possible?

Laser action is usually described in terms of photons and stimulated emission. In 1972, Borenstein and Lamb published a paper* claiming that lasers can be described classically on the basis of nonharmonic oscillators. Is this approach or any other classical approach adequate to explain laser operation ? Is a classical description of lasers possible?

Some new classical results are here: https://phys.org/news/2020-07-laser-spectral-linewidth-classical-physics-phenomenon.html

*No promotion of the site is intended. It is convenient to display the reference and the abstract.

• Your title and question body are somewhat of a mismatch. Do you want the approach/paper explained, or to know the present view of this approach compared to other literature and progress that has been made since? – JamalS May 27 '18 at 12:56
• The question is : Is a classical description of laser operation possible? – my2cts May 27 '18 at 13:32
• Does your paper not answer that? – JamalS May 27 '18 at 14:32
• The conclusion in abstract that I linked to is "laser action is not in- trinsically a quantum-mechanical effect". So according to Lamb the answer is yes. As a Noble laureate he deserves some credit. However all treatments I know invoke quantum mechanics and also "laser" is an acronym for "light amplification through stimulated emission". – my2cts May 27 '18 at 14:45
• Part of the answer is found here physics.stackexchange.com/questions/81102/… : it is stated that the Free Electron Laser can be described completely by classical electrodynamics. – my2cts May 28 '18 at 19:03

Quantum mechanics is needed before the ionization of the electron to properly describe a laser.

See "Introduction to Quantum Features of Laser Physics" (Phys. Scr. 1986) Stig Stenholm for a single page recap of the historical description of lasing, and a short list of reference articles.

For an explanation that is as classical as possible see: "From a quantum to a classical description of intense laser–atom physics with Bohmian trajectories" (20 Nov 2009), by Lai, Cai and Zhan:

Abstract.

In this paper, Bohmian mechanics is applied to intense laser–atom physics. The motion of an atomic electron in an intense laser field is obtained from the Bohm–Newton equation. We find that the quantum potential that dominates the quantum effect of a physical system becomes negligible as the electron is driven far from the parent ion by the intense laser field, i.e. the behavior of the electron smoothly tends towards classical soon after the electron is ionized. Our numerical calculations present direct positive evidence for semiclassical trajectory methods in intense laser–atom physics where the motion of the ionized electron is treated by classical mechanics, while quantum mechanics is needed before the ionization.

...

The only difference between the Bohm–Newton equation and the Newton equation is that there is an extra term in the Bohm–Newton equation called the quantum potential. When the quantum potential is negligible, the Bohm–Newton equation will reduce to the standard Newton equation and then the motion of the particles can be described by classical mechanics.

See also our question: Laser beam in terms of maxwell's equations

• This doesn't seem to relate to what the OP is asking. The OP is asking about the lasing process itself, not the ionization of an atom by a laser. – user4552 Jun 3 '18 at 17:07
• First sentence: en.wikipedia.org/wiki/Ion_laser – Rob Jun 3 '18 at 17:34

you can certainly explain a large amount regarding laser action with a classical EM field. However, to create gain and to compute the emission rates (spontaneous and stimulated) one would have to use basic quantum wave mechanics as Einstein in his famous 1917 paper describing such action.

• If that is true then where does Lamb go wrong? And what is the fundamental difference between a free electron laser producing coherent 10.6 $\mu m$ light and a CO2 laser producing the same ? – my2cts Jun 5 '18 at 18:25