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04-30-2015, 07:56 AM #1
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Understanding divergence in DPSS and Laser Diodes

I am trying to understand why single mode laser diodes have lower divergence after collimation than multimode laser diodes. From reading the following quote from another discussion group, something must be missing in my understanding. I thought the smaller the emitter the less divergence but the following seems to indicate the opposite:

Quote:
 Due to Heisenberg uncertainty principle ΔxΔp≳ℏ2 , one can't really make a quantum have zero momentum in any direction. So you can't say that photons go in the same direction - this is just a simplified description of laser operation. In reality, the thinner the beam, the higher the divergence. Compare e.g. a DPSS laser (e.g. green laser pointer) with a diode laser (e.g. a red laser pointer). •In a DPSS laser the active material will have diameter of order of hundreds of micrometer, and the exiting beam will start from even smaller diameter for various reasons. The divergence is quite small: if you remove the collimating lens, your light image from a green laser pointer will be several centimeters after the light goes several meters. Divergence angle would be ∼λ/d=532nm/100μm∼0.3 ∘ . •If you try doing the same with a red laser pointer, you'll see that its light diverges quite a lot: after going several centimeters in direction of propagation, it'll already give image of several centimeters. The reason for this is that active zone of diode laser has diameter of order of several micrometers. This makes output beam quite thin, making Δx small and thus Δp high, and this is what leads to high divergence. Divergence angle would be ∼λ/d=640nm/1μm∼40 ∘ . Actual angle would depend on which transverse direction you select, because active zone is ∼10× longer in one direction than in another. In general, the thicker your starting laser beam, the more collimated it is, so if you manage to make a (visible wavelength) laser with beam starting at 1cm thickness, you'll have almost perfectly collimated laser beam.

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04-30-2015, 09:07 PM #2
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lazeristasUVISIR
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Re: Understanding divergence in DPSS and Laser Diodes

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04-30-2015, 09:36 PM #3
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Re: Understanding divergence in DPSS and Laser Diodes

Is this it?

The reason single mode laser diodes have less divergence than multimode diodes is because the smaller the emitter, the more relative beam expansion there is before collimation, compared to multimode emiiters which are so much larger to begin with?

Another way of saying the same thing: Is this merely a ratio of expansion which makes single modes with their tiny emitters produce less divergence, since the beam must travel a distance before collimation, thus the ratio of expansion greater?

Or, is it due to the structure of the waveguide output of a single mode diode optically producing less spreading in one axis due to the dimensions of the emitter?

Not sure I have a handle on it yet.
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Sincerely investigate any of these three short quotes as new concepts and you've taken your first step into a larger world:

"I regard consciousness as fundamental. I regard matter as derivative from consciousness..." - Max Planck. "Everything we call real is made of things that cannot be regarded as real" - Neils Bohr. "What we call physical things and events do not exist independently of subjective experience..." - Deepak Chopra.

Each of these three rabbit holes go deep, ending up in the same place.

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05-01-2015, 12:02 AM #4
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Re: Understanding divergence in DPSS and Laser Diodes

That's comparing apples and orange mummified armadillos. If you expand the diode's output in the example to that of the green's output, the divergence would be similar.

You're just confusing beam expansion with emitter size. A larger emitter size is conducive to multiple modes.
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05-01-2015, 12:48 PM #5
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lazeristasUVISIR
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Re: Understanding divergence in DPSS and Laser Diodes

Quote:
 Originally Posted by Cyparagon That's comparing apples and orange mummified armadillos.
Enough said.
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