Old 01-26-2011, 04:32 AM #1
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Default What makes a Laser Diode different from an LED?

So yeah, that's basically what I'm wondering. What distinguishes a laser diode from an LED?

Both are diodes that emit light of a specific wavelength when energized.

The light from a laser diode is not focused into a beam without the aid of a lens.

Could someone take one of these bright CREE LEDs, use a lens to focus it's output, and build a "white laser"?


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Old 01-26-2011, 04:56 AM #2
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Default Re: What makes a Laser Diode different from an LED?

Quote:
Originally Posted by Bogart View Post
So yeah, that's basically what I'm wondering. What distinguishes a laser diode from an LED?

Both are diodes that emit light of a specific wavelength when energized.

The light from a laser diode is not focused into a beam without the aid of a lens.

Could someone take one of these bright CREE LEDs, use a lens to focus it's output, and build a "white laser"?

The primary distinguishing factor of a laser diode, is Coherent light and monochromatic light.

Coherent light implies that the amplified radiation output of a diode is in phase, unlike regular light. this is what allows you to cram so much power into a small area, and makes effective collimation possible.
This has to do with phasing and how laser light gets amplified to have such a high power density, where destructive interferance due to out of phase output from an LED would make this near impossible.

Comparing this as a transverse wave, the output is in a position where amplification is possible as it "lines up" as constructive interferance, amplifying the emittion. an LED will not do this.

Monochrome light bieng the output, is very pure for its wavelenth, where it emits a single wavelenth, rather then variant harmonics that LEDs emit along.

Using a white LED to make a white laser will not work for the following reasons:

Incoherent and multiple wavelenth output: Like it or not, white is going to be a full mix of wavelenths, main stopping factor is so much interferance and different phase and harmonics, it pretty much makes it impossible.

You can make a white laser out of an RGB laser setup though, where you use the polarity of the output and combine the beam this way. as your lasers are coherent, you can play with the polarity and effectivly combine beams together to our perception of white light, and retain this collimation as a form of a laser beam.

Second:
White LEDs are achived by a blue LED that has a phosphorous coating, so even then its not a direct emitter source, but rather just the phosphorous bieng excited into emitting light.

You will often hear of a laser diode going LED, this event happens when the diode undergoes catastrophic optical damage, (to the facet I think) and no longer outputs amplified radiation in phase as its supposed to, leaving you with a really expensive LED instead of a laser source.

Hope that answered your question, But Id like some more knowledgeable members to point out any errors I could have made. So if you spot a mistake, call it out please, thats how I can learn.

Last edited by GBD; 01-26-2011 at 05:28 AM.
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Old 01-26-2011, 05:05 AM #3
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Default Re: What makes a Laser Diode different from an LED?

Laser is an acronym. The A is for amplification. There is no amplification of light in a LED. LEDs also emit over a much wider band than you might think.

5mm white LED, 5mm red LED, and red 1mW pointer, respectively:





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Old 01-26-2011, 05:17 AM #4
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Default Re: What makes a Laser Diode different from an LED?

This kind of science should be more common on LPF.
Thanks for the break from appliance postings.
When a laser diode is overpowered to COD, the optical cavity is damaged by heat or power and amplification stops, making it a basic LED.
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Last edited by Hemlock_Mike; 01-26-2011 at 05:22 AM.
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Old 01-26-2011, 05:37 AM #5
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Default Re: What makes a Laser Diode different from an LED?

Wow, great answers. That makes a lot of sense...the whole destructive interference thing.

I guess multimode diodes like these 445s are able to achieve coherence even with their multiple emitters huh...

I remember hearing how Nikola Tesla was very much into constructive interference and resonance and such things. The idea of finding a body's resonant frequency, imparting energy into it, and then adding energy at the moment the initial burst returns to again bounce back.
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Last edited by Bogart; 01-26-2011 at 05:42 AM.
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Old 01-26-2011, 05:42 AM #6
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Default Re: What makes a Laser Diode different from an LED?

Good old tesla.

Something that I was doing before I got seriosly involved with lasers again, was making tesla coils (both spark gapped and the solid state counterpart), but the concept remained the same, voltage gain via resonance, rather then inherently depending on the turn ratio of the primary VS secondary as conventional transformers operate.

The solid state version of the tesla coils gets rid of the lossy spark gap (that switches the LC circuit tuned to the primary and secondary), and replaced it with solid state devices, it all makes it more efficient.
So tunning for resonance is critical for this oscillator, else you waste alot of power and dont gain much output.
(Solid state is also has its advantage, you can use a feedback circuit to keep it at its resonant freq)

Alot of our modern technology wouldn't exist if it wasnt for tesla.
Poly phase power transmission, radio (not that marconi fake), resonant transformers, AC motors, bladeless turbine and many more.
(I think he had 300 something patents)

Now that the offtopic bit is over, I really would like someone to re-read what I wrote in the second post and comment on its accuracy please. Im fairly sure that how it happens, but Im still new to how light will behave, so its good to have some feedback.

Last edited by GBD; 01-26-2011 at 05:53 AM.
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