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I've found an SE question (Laser without a cavity) about that, but it isn't exactly the same as mine.

My thinking is: no, you can't, because without the cavity, there wouldn't be possibility for one dominant phase to take over and create stimulated emission for that particular phase only. Best you could achieve is just very strong, kinda-monochromatic light amplification. Am I right?

But what would be other properties of such light source? Would it create a beam? I don't think so, because lack of mirrors would make it impossible for one direction photons to take over the stimulated emission. Is that correct?

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    $\begingroup$ A practical nitrogen laser does not need a resonant cavity. C. L. Strong published plans for a D.I.Y. version many long decades ago in his "Amateur Scientist" column in "Scientific American." Sorry, but I couldn't find a link right away, and don't have any more time to spend right now. $\endgroup$ – Solomon Slow Apr 12 '20 at 12:36
  • $\begingroup$ As one of my early collaborators said to me, pump anything hard enough and it will lase... $\endgroup$ – Jon Custer Apr 12 '20 at 16:01
  • $\begingroup$ Formatting note: acute accent ´ shouldn't be used instead of apostrophe '. I don't know what your keyboard layout is, but on the standard US layout apostrophe is near the Return (Enter) key, on the same key as quotation mark ". $\endgroup$ – Ruslan Apr 13 '20 at 8:48
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I think you basically answer the question yourself and @mikestone seems to make the same point: you can have strong light amplification without cavity. However:

  • This is an inefficient use of the active media
  • You will have no control over the modes, including the direction of the emission.

Whether one may call this a laser/maser or simply and amplifier is a matter of terminology.

Remark Terms laser/maser (light/microwave amplification by stimulated emission of radiation) are inherently ambiguous. This is why one would sometimes talk about quantum generators as opposed to quantum amplifiers, in analogy with the traditional generators and amplifiers, where the former are characterized by a periodic cycle (like a photon bouncing back and forth between the mirrors).

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  • $\begingroup$ -1, I have to disagree with this answer on all fronts. As a counter-example to your points, in my lab we use these laser diodes very frequently without a cavity. They amplify light by stimulated emission of radiation, so in my view it's not ambiguous to call them lasers. They have directed outputs which you control by having a good input mode & coupling, and it's a perfectly efficient use of the gain medium. $\endgroup$ – aquirdturtle Apr 13 '20 at 8:38
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    $\begingroup$ @aquirdturtle What do you disagree with? Terms laser/maser are used both for generators and for amplifiers, as you yourself note. Thus, the meaning of these terms is ambiguous. Specifically regarding the laser diodes: the crystal surface reflects at least 30% of the radiation, and many would say that it constitutes a cavity. $\endgroup$ – Roger Vadim Apr 13 '20 at 8:44
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    $\begingroup$ @aquirdturtle these devices actually aren't supposed to lase. This article explicitly mentions that "For optimized performance, the tapered amplifier design can contain additional elements, such as cavity-spoiling groves approaching the transition region between ridge waveguide and taper region, which can suppress parasitic lasing.". This makes cavity an essential feature of an artificial laser. $\endgroup$ – Ruslan Apr 13 '20 at 9:03
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    $\begingroup$ @aquirdturtle I think you're just fixed on the term initially being an acronym. Nowadays technology does distinguish lasers and amplifiers. But, actually, I see now in this article that although generally a laser requires a cavity to compensate for losses, "exceptions are a few cases where a medium with very high gain is used, so that amplified spontaneous emission extracts significant power in a single pass through the gain medium." So there indeed are a few kinds of lasers without cavities (although I've not found any concrete examples) $\endgroup$ – Ruslan Apr 13 '20 at 20:20
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    $\begingroup$ @Ruslan Yeah, at some level this ends up just sounding like semantics. Although as I said, the tapered amplifiers I use in the lab are a concrete example. Astrophysical lasers also exist and are cool examples. I think my frustration with the topic is mostly tied to other more meaningful misconceptions about lasers which seem to stem from this. For example, that lasers have to be coherent, which they don't. They just usually are (coherence is relative anyways). $\endgroup$ – aquirdturtle Apr 13 '20 at 21:03
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Masers, and even optical frequency lasers, occur in molecular clouds. These are single-pass processes rather than resonant cavities and so not always regarded as a "true" laser. See: https://en.wikipedia.org/wiki/Astrophysical_maser

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Congratulations, you've invented one widely-used type of https://en.wikipedia.org/wiki/Optical_amplifier. :P Wikipedia has quite a bit of stuff about uses for stimulated-emission without a cavity, including amplifying the output of an existing laser (e.g. as a laser guide star as a reference for adaptive optics in a telescope).

Apparently some designs (like a semiconductor optical amplifier) that are naturally built a lot like a laser diode would be have to intentionally avoid reflections to not create a cavity. Even so, amplified spontaneous emission is still a source of noise when your goal is to amplify existing signals. Non-linearity of semiconductor amps can apparently be used for optical signal processing.

It's not in general limited to amplifying a single frequency, though. Real life repeaters for long-haul fiber-optic data links use this to amplify all the light of different wavelengths in a fiber carrying different signals on different wavelengths, e.g. 1310 nm and 1550 nm or a range of wavelengths around those. (WDM = Wavelength-Division Multiplexing; like using a prism to get signals from different modulated lasers in, and another prism (or actually etalons) on the receiving end to split it up for different detectors.)

Optical repeater amplifiers are used to boost signal strength in long-haul fiber optics data links (under-sea / between cities). Every few hops you need to clean up the signal (because of modal dispersion even in single-mode long-haul fiber and other effects that cause a pulse to spread out in time). So every few hops you use a more expensive repeater that digitizes the incoming signals (on each wavelength) and then modulates an output laser. Or maybe something clever that doesn't go all the way back to electrical.

Wikipedia's Optical Amplifier article starts off:

... An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiberoptic cables which carry much of the world's telecommunication links.

There are several different physical mechanisms that can be used to amplify a light signal, which correspond to the major types of optical amplifiers. In doped fiber amplifiers and bulk lasers, stimulated emission in the amplifier's gain medium causes amplification of incoming light. ...

(This doesn't imply that the only thing you can do with a non-cavity laser-like thing is amplify optical signals. But if you do want to amplify optical signals, stimulated emission is your best bet.)

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