Derived from the Acronym L.A.S.E.R. : Light Amplification by Stimulated Emission of Radiation. Name of a device or process that amplifies radiation. Cooperative emission of photons from an inverted population, usually defined as a device in which more stimulated emission is occurring than spontaneous emission. (some authors have a stricter definition of LASER; which includes a resonant cavity as a pre-requisite) Although light usually refers to optical radiation, it can also mean any frequency of electromagnetic radiation. The habit of calling some of the very first lasers : Optical MASERs now seems somewhat backwards, therefore for clarity we shall use the following naming convention : Each photon can stimulate the emission from an excited state. The resulting two photons are exact copies. Each of those two photons can stimulate an additional two photons, thus we obtain 4 identical photons. If this process is repeated a dozen times as the photons propagate along the length of the active medium, the signal can cause a cascade chain-reaction multiplication by a factor of 10,000. Since each stimulated photon inherits its parents' attributes, a single photon can potentially give birth to thousands of identical clones of itself; the laser is nothing more than a photon cloning machine with the feedback potential of producing what Dicke (1956) called an optical bomb.

Dicke (1954, 1964) introduced the concept of superradiance, and later referred to this effect as an Optical Bomb because of the unusually short and intense light burst from a chain-reaction cascade of photons from stimulated emission. (p.103, 113, 144) He was the first to treat correlated emission of radiation from a system of excited atoms :

Since the photon is both a particle and a wave, the atoms are interacting with a common electromagnetic field created by all the photons. The system of radiating atoms must be treated as a whole and not as separate isolated spontaneous photon emissions. The common picture that each photon as a little particle that can stimulate other excited atoms it encounters is a somewhat inaccurate particle viewpoint. It ignores the wave aspect, each photon's wavefunction can be spread over the entire lasing medium, therefore each atom does not radiate independently of each other. Only by considering the lasing medium as a single quantum mechanical system can the correct behavior of the laser be predicted.

When the lasing medium is not contained within a cavity, the dominant modes of correlated photon emission are

  1. Superfluorescence
  2. Amplified Spontaneous Emission
  3. Superradiance
Normally a metastable level would decay by spontaneous emission of a photon or by other non-radiative decay mechanism after a sufficient time interval has elapsed. However when the population of this metastable state exceeds the ground state, stimulated emission begins to dominate over spontaneous decay as the de-population mechanism. If the lasing medium is contained within a Fabry-Perot type of resonant cavity, and if the radiation intensity is larger that a well defined threshold where gain exceeds loss, then almost all of the excited ions decay prematurely by stimulated emission.

This photon cascade contributes to a macroscopic electromagnetic cavity mode or quantum wavefunction consisting of an enormously intensified copy of the original first few spontaneous emissions that sparked the initial cascade.

If the lasing medium is contained within an optical cavity the repeated passes of the stimulated emission output can enhance the gain sufficiently to create a macroscopic electromagnetic mode which can build up to produce a highly coherent, narrow output beam of extreme monochromaticity : The spectral width of the emission line can narrow by four orders of magnitude over spontaneous fluorescence or amplified spontaneous emission. In this regime of operation, a strange oscillatory behavior of the output pulse can be observed, which is attributable to an accelerated rate of stimulated emission which drives the inverted population below threshold, the lasing medium the requires a period for the populations to build up above threshold once again.

If stimulated emission dominates over spontaneous emission we have amplified spontaneous emission or superradiance. If the ions are placed within a cavity, the gain can be significantly improved to the point where the output beam becomes coherent, extremely narrow and of significantly reduced spectral width, essentially producing a single resonant mode of electromagnetic radiation.

The effect of the cavity is not only to maintain a large enough electromagnetic field strength to stimulate emission from excited ions but also to maintain feedback and thus coherence of the output beam. The optical cavity serves improve gain by multiple passes and to create a monochromatic output beam by using Fabry-Perot mode selectivity.


In a lasing medium with a population inversion (to any degree), since the lasing medium acts like an amplifier it will amplify any radiation at the precise wavelength of the laser transition. This means that photons not falling with the Doppler broadened gain profile will most likely pass through the plasma without being causing a stimulated emission. Which means that only a small fraction of the total blackbody energy produced near the lasing medium will be successful in producing emission. Photons that produce the quantum transition by spontaneous emission have a much better chance of being amplified because they are more numerous than stellar photons at that particular wavelength that those from the nearby blackbody, and so the emission line laser radiation from quasars is mostly from amplified spontaneous emission rather than amplified photospheric emission.

Spontaneous emission is considered random, and getting more light from a blackbody is difficult. You can't make it thicker because it tends to absorb the radiation it emits, it is optically thick. You can raise the temperature, or you can increase the effective surface area because at any given temperature a blackbody emits a fixed amount of energy per unit surface, no more no less.

H II regions are nebulas that are often optically thick only at discrete wavelengths hence, a lot of ultraviolet radiation can penetrate the volume of the gas and photo-ionize it, but again, the nebula can't emit more light of a certain wavelength than a theoretical surface enclosing the same volume because of optical thickness.

When laser action is taking place, this self-limiting effect is no longer operating. Because of the possibility of longer path lengths through the plasma, the larger the volume of active lasing medium, the greater the gain. In the laser, stimulated emission from a population inversion creates a negative optical thickness. When natural microwave lasers where discovered, the strongest emission usually occurred from the longest path with the largest negative optical thickness. This is contrary to what one would expect from large positive optical thicknesses in spontaneous emission lines.


  1. Dicke,R.H.: 1954, Phys.Rev., 93, 99.
  2. Dicke,R.H.: 1964, The Coherence Brightened Laser in Quantum Electronic eds. Grivet,P., Bloembergen,N p.35
  3. Gordon,J.P., Zeiger,H.J., Townes,C.H.: 1954, Phys.Rev., 95, 282.

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