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In what sense the laser beam is monochromatic if there are multiple laser transitions happening in the active medium? For example, The two main argon laser transitions are at visible wavelengths:

  • Blue 0.488 [μm],
  • green 0.5145 [μm],

And still, it has laser transitions in the UV spectrum:

  • 0.3511 [μm]
  • 0.3638 [μm]

If for the sake of the argument, one ignores the natural and the Doppler noise and the one due to atomic collisions, I wouldn't still get a pure wavelength since there are multiple laser transitions.

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    $\begingroup$ Note that you'll never get complete monochromaticity at least because of natural broadening $\endgroup$
    – Ruslan
    Mar 12, 2023 at 12:39
  • $\begingroup$ IDK about lab lasers, but argon ion lasers often were used in laser light shows back in the days before DPSS lasers were cheap and plentiful* precisely because a single laser produced a beam that could be split into several different colors. [* I also DK what they use in modern light shows.] $\endgroup$ Mar 12, 2023 at 13:49
  • $\begingroup$ @SolomonSlow - although in the lab one used additional optics in the cavity to select one line, fiddling with it to get another line if you wanted to change wavelengths. Just a question of how broad or narrow the cavity selectivity. $\endgroup$
    – Jon Custer
    Mar 12, 2023 at 15:07
  • $\begingroup$ To demonstrate the type of optics @JonCuster may have fiddled with: I've seen air-cooled argon ion lasers with selection filters on the output, which were easy. The water-cooled one I resurrected (20kW in for 250mW at 488) had a prism in the cavity, that had to be tuned to the wavelength at which I wanted the gain, which of course had to match one of the gas's emission lines. $\endgroup$
    – Chris H
    Mar 13, 2023 at 16:19
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    $\begingroup$ 488nm (cyan) and 514nm (green) are comparable in brightness and far brighter than anything else from argon (458 is a better blue but quite weak in comparison) . There's a red krypton line, and some tubes were built with an Ar/Kr mix to get RGB emission, though never really very well at the same time. (@SolomonSlow) $\endgroup$
    – Chris H
    Mar 13, 2023 at 16:21

2 Answers 2

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Lasers are monochromatic in the sense that they produce a narrow range of frequencies.

The difference between a laser and an ordinary light is the optical cavity. Cavity design is complex. For some lasers a very narrow range of frequencies is crucial. Others have different goals.

Cavities have modes. A mode is a beam that can reflect one round trip through the cavity and interfere constructively with itself. This more or less means double the cavity length must be a multiple of the wavelength.

For cavities longer than a laser diode, the natural line width of a given transition is typically much larger than the width of a mode. This means that the laser could emit several closely space frequencies. Mode competition can prevent this. An atom in a cavity stays in an excited state until a photon comes along and stimulates the atom to emit an identical photon. If one mode has a high intensity than the others, the odds are highest that the next photon to pass by will be from that mode. That mode gains intensity at the expense of others, and quickly becomes the only mode.

One transition may have a higher gain than another. This transition will be the one to lase. This may not be your favorite transition. Sometimes an optical filter is put in a cavity to absorb this wavelength and prevent it from being amplified.

This is enough for most lasers. But some, designed for Single Frequency Operation, achieve a narrower line width. One way is to add an optical filter, such as an etalon, that only passes an extremely narrow band of wavelengths. There are other approaches.

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  • $\begingroup$ Presence of cavity is the difference between a generator and amplifier of radiation. What differs laser radiation from conventional light sources is coherence - in time and space, which is not true for chaotic light. $\endgroup$
    – Roger V.
    Mar 13, 2023 at 14:56
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One can consider a given laser as a device consisting of three elements: a gain medium, a pump, and a cavity. The laser cavity acts as a frequency (color) selection element. The only light that is getting amplified inside a laser must be resonant with the cavity. Thus the quality of your cavity determines how monochromatic is your radiation. Cheap laser diodes can have broader than $1$ nm (few THz) spectra when a decent scientific laser has a spectral width below $1$ MHz.

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  • $\begingroup$ $>1\,\mathrm{nm}$ sounds like a diode below lasing threshold. $\endgroup$
    – Ruslan
    Mar 12, 2023 at 11:44
  • $\begingroup$ @Ruslan Yeah, for example, old DFBs are like that. The first link from google with a spectrum: laserdiodesource.com/dfb-lasers $\endgroup$
    – Noct
    Mar 12, 2023 at 11:50
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    $\begingroup$ @Jack For laser based on argon, as in your example - yes. For a laser based on a solid state gain medium where you have continuous emission spectra, what is amplified is a section of that spectra matching your cavity. $\endgroup$
    – Noct
    Mar 12, 2023 at 11:58
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    $\begingroup$ @Jack Not a whole spectrum, just a section. Monochromaticity is a subjective term. There are cases when a spectrum of 100 kHz is not enough (for example frequency standards) and cases when you are fine as long as your optics works as designed for a central wavelength (industrial lasers). If your laser is significantly narrower than your gain medium you can call it a success. $\endgroup$
    – Noct
    Mar 12, 2023 at 12:13
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    $\begingroup$ @RonJohn It's true that technical difficulties are almost always a limiting factor. However, it is good to know that there is also a fundamental limit coming from quantum fluctuations. So a lack of mathematical exactness is also a part of nature itself. $\endgroup$
    – Noct
    Mar 13, 2023 at 0:25

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