Laser (source code)
= Laser
{wiki}
A laser generates amplified <light> through <stimulated emission>. Its name abbreviates “light amplification by stimulated emission of radiation”. Lasers supply the intense beams used in <optical tweezers>; <optical coherence> also makes them useful for <wave interference> measurements. https://openstax.org/books/university-physics-volume-3/pages/8-6-lasers[OpenStax University Physics 3, §8.6: lasers]{ref}.
In the usual material picture, a gain medium stores <energy> supplied by pumping, for example by incident <light> or an <electric current>. <Stimulated emission> transfers that stored <energy> to a selected optical <normal mode>, while competing <photon absorption> removes <light>. Producing net <optical gain> requires a suitable population distribution, such as <population inversion> in the simple model below.
A familiar <laser> oscillator uses mirrors to return <light> through the gain medium. This optical feedback favours resonant <normal modes>; a partly transmitting mirror lets useful <power> escape. Feedback and mode selection have different implementations, including ring arrangements: every <laser> need not resemble the same two-mirror device. The <laser threshold> example treats just that device. https://ocw.mit.edu/courses/res-6-006-video-demonstrations-in-lasers-and-optics-spring-2008/37b4c2aa5a6e3e0c119688024ce90ce5_Iqp7NxnwaGY.pdf[Shaoul Ezekiel, MIT, Laser fundamentals I: Simple laser, transcript]{ref} https://ocw.mit.edu/courses/6-974-fundamentals-of-photonics-quantum-electronics-spring-2006/18f30fad63a62ef4dd894d3752b55a60_chapter7.pdf[Franz Kärtner, MIT 6.974, Chapter 7, §7.1: laser oscillator concept]{ref}.
The useful combination is concentrated <power> and controlled optical <normal modes>. It gives <optical tweezers> a localized source of <force> and <wave interference> measurements a source with useful <optical coherence>. A <laser> can nevertheless emit several <frequencies>; “laser” alone does not guarantee a single <frequency>. https://ocw.mit.edu/courses/res-6-006-video-demonstrations-in-lasers-and-optics-spring-2008/674c0c322cf0d87072e31de3e51f8e27_--Zi_cn4kPE.pdf[Shaoul Ezekiel, MIT, Laser fundamentals I: Spectrum of laser light, transcript]{ref}.
An original experimental landmark is T. H. Maiman's 1960 report of stimulated optical radiation from optically pumped ruby at about $694.3\,\mathrm{nm}$. His paper describes both the pumping path and the observed emission, connecting stored material <energy> with amplified <light>. <Stimulated Optical Radiation in Ruby by Theodore Harold Maiman>[T. H. Maiman, Stimulated Optical Radiation in Ruby, Nature 187, 493–494, 1960, original paper]{ref}.
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