I've been reading about how light is produced in both laser diodes (the most common types of lasers) and LED's.

The mechanism seems very similar if not exactly the same: one applies a potential difference through a semi-conductor material that emits light when the electrons recombine with holes in a p-n junction.
Thus I would have expected the broadness of the spectrum of both laser diodes and LED to be similar. However they aren't. Here's the spectrum of some LED's: https://upload.wikimedia.org/wikipedia/commons/f/f4/Red-YellowGreen-Blue_LED_spectra.png.

While the spectrum of a laser can be found there: http://ondax.com/wp-content/uploads/2011/01/chart.jpg or there: http://ledmuseum.candlepower.us/rlspec4.htm.

We can see that the spectrum of a laser diode is much narrower in terms of wavelengths than that of a LED's one. I'd appreciate if someone could tell me why it is so or any reference where I can read the reason.


  • $\begingroup$ IANAP. Isn't it just that the resonance set up within the laser cavity that prevents the wider frequencies from becoming coherent (and thus lasing) ? $\endgroup$
    – Alnitak
    Commented Mar 31, 2015 at 16:33
  • $\begingroup$ That's the first thing that came to my mind but I was told that no, on IRC in the #physics channel. So I am not sure at all. $\endgroup$ Commented Mar 31, 2015 at 17:05
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    $\begingroup$ LASER= light amplification by stimulated emission of radiation. You probably pump by the diode light, but the light you get out is a result of an avalanche from one resonant state - an emission stimulated by other photons of the same resonant energy. $\endgroup$
    – jaromrax
    Commented Mar 31, 2015 at 17:41

1 Answer 1


When you first apply current to a laser diode, it does behave as an LED. Light is output across a (relatively) broad spectrum by spontaneous emission.

But once the current reaches the threshold current, then positive feedback causes one (or a few) modes to oscillate. Further increases in input power will increase the ouput in those particular modes, but the gain will be pinned at the threshold level. Since gain is mainly a function of carrier density, this means the carrier density is also pinned, and spontaneous emission won't increase.

Similar gain-pinning mechanisms are seen in most laser systems.

  • $\begingroup$ Am I right if I say that for LED's the current does not reach that threhold reached by laser diodes? $\endgroup$ Commented Mar 31, 2015 at 18:58
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    $\begingroup$ @no_choice99: In principle yes. But I would use the slightly different wording "A laser below threshold behaves as an LED" (because a LED has no threshold). $\endgroup$
    – Andreas H.
    Commented Mar 31, 2015 at 19:54
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    $\begingroup$ @no_choice99, the LED can't reach threshold and start lasing because there's no (or not enough) optical feedback (high-reflectivity mirrors) like there is in a laser diode. $\endgroup$
    – The Photon
    Commented Mar 31, 2015 at 20:30

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