Hi
Could anyone tell me the difference between a LED and a laser diode.
I know that LEDs produce light via spontaneous emission that occurs when electrons and holes recombine in a PN junction and that laser diodes work via spontaneous emission but what is the difference in construction?
Thanks
Mike
Hehe. I'm sure you'll get various, more serious answers. But from a practical optics standpoint I sometimes imagine they aren't so much a laser as they are a somewhat narrower band LED. Some spatial and temporal coherence, but only enough for a meter or two. :) Polarization is something like 100:1, I think.
Jon
In a laser diode, excited bits in the semiconductor matrix ("excited" by having their electrons "boosted" from the "valence band" to the "conduction band") mostly do not release their stored energy as photons spontaneously, but do so in response to stimulation by similar photons (this is "stimulated emission").
In a laser diode, the "stimulated emission" results in the photons hving coherence. For one thing, the bandwidth of a usual laser diode is around or less than 1/10 naometer, while that of regular LEDs is a few to a few
10's of nanometers.The light of a laser diode is polarized while that of a regular LED is not.
Another thing: The light emitting surface of a usual laser diode is of roughly bacterium size - a few to several micrometers by a micrometer or two. The light emitting surface of most regular LEDs is 200-300 micrometers wide. The small size of the light emitting surface of most laser diodes results in a beam that has significant divergence despite the coherence. As a result, laser pointers have lenses that collimate the beam into one with low divergence. The same lens cannot do the same with ordinary LEDs, due to the larger light source size with lack of coherence.
- Don Klipstein ( snipped-for-privacy@misty.com,
Excellent point. I had forgotten to add it!
Also, do you know if, by addition of appropriate external cavities, laser diodes can maintain their coherence over longer ranges than just a few meters?
Jon
hi, laser light is a coherent beam of light, led light is spread out : non coherent. mark k
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