Difference between LED and laser diode?

[mike]

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

 

[Jonathan Kirwan]

On 8 Sep 2004 11:33:49 -0700, mike_uk31415@yahoo.co.uk (mike) wrote:

Could anyone tell me the difference between a LED and a laser diode.

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

 

[Jonathan Kirwan]

On Thu, 9 Sep 2004 01:13:58 +0000 (UTC), don@manx.misty.com (Don Klipstein)
wrote:

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.

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

 

[Don Klipstein]

In article <8ccca585.0409081033.6804bbcf@posting.google.com>, mike wrote:

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?

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 (don@misty.com, http://www.misty.com/~don/laserdon.html)

 

[mark krawczuk]

hi, laser light is a coherent beam of light, led light is spread out : non
coherent.
mark k

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"mike" <mike_uk31415@yahoo.co.uk> wrote in message
news:8ccca585.0409081033.6804bbcf@posting.google.com...

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