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Difference between LED and laser diode?

Discussion in 'Electronic Basics' started by mike, Sep 8, 2004.

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  1. mike

    mike Guest

    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
     
  2. 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
     
  3. 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 (, http://www.misty.com/~don/laserdon.html)
     
  4. 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
     
  5. hi, laser light is a coherent beam of light, led light is spread out : non
    coherent.
    mark k

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