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Old 08-27-2008, 02:10 PM #17
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Default Re: DPSS process

Quote:
Originally Posted by FrothyChimp
for a hobbyist a diode is fine but what is the beam quality (M[sup]2[/sup]) value of a 671 DPSS versus a red diode? 1.2 versus about 20. Beam quality generally sucks using a diode.
Quote:
Originally Posted by hydrogenman15
Yeah,but let just say the focus on my red is as perfect as it can get. I was under the impression that red had BETTER divergence then DPSS's. :-?
Because of diffraction, a smaller output aperture yields a greater divergence. HeNe tubes had apertures made from concave output-coupler mirrors that were, what, a couple of 16ths of an inch or so across? (Dave?) The optical cavity of a diode laser is basically a microscopic scratch on a semiconductor. This makes the light waves come out much like the waves in the 'double-slit' experiment - they 'wrap around' the edges of the slit. They do this in both directions, but since the scratch is much shorter than it is wide there is greater diffraction in the vertical direction. This contributes to the non-circular shape of a laser diode's spot... generally the diffraction in the vertical direction is about 3X that of the horizontal direction.

In a DPSS laser, the light from the diode is used to excite crystals that then emit the light. The size of this crystal is quite large compared to the aperture of the diode. This contributes to the smaller diffraction and therefore smaller divergence of the output beam. So no, a red diode does not generally have a better divergence than a DPSS laser. A red HeNe, on the other hand, will most likely have better divergence than either the diode or the DPSS laser.

(Side note: I think diffraction is just a facet of the Heisenberg uncertainty principle... it just seems like the harder we try to pin down one variable the more squirrelly the others get, any time we're anywhere near Maxwell's equations.)

Different colors diffract by different amounts. This is one of the reasons why the double-slit experiment needs monochromatic light: otherwise, the diffraction patterns of each wavelength would smear into each other, fuzzing out the results. (Refraction has this property as well... you could even define colors as being wavelengths that refract by a specific angle. This isn't voodoo, it's basically just solving Maxwell's equations for different variables).

The M[sup]2[/sup] value FrothyChimp speaks of is a measure of beam quality that doesn't depend on wavelength, so we can use it to compare lasers of different colors without getting hosed by the different diffraction angles. There's a value called the beam parameter product (BPP) that is just the divergence multiplied by the radius of the narrowest point of the beam (the beam waist). The lowest-possible BPP comes from a beam with a Gaussian profile (which you might have seen called TEM[sub]00[/sub]). M[sup]2[/sup] is just the ratio of your beam to that ideal TEM[sub]00[/sub] beam, i.e. you can focus your laser to a spot whose area is M[sup]2[/sup] times as large as that of a similar TEM[sub]00[/sub] beam. (So smaller numbers are better, and you can't get better than M[sup]2[/sup]=1.)

================

On a semi-related note: look at a prism. (If you don't have a prism, the cover of Pink Floyd's Dark Side of the Moon will do.) (If you don't have Pink Floyd's Dark Side of the Moon, may the gods have mercy on your soul.)

OK... note that red 'bends' less than blue. All the colors in between bend by proportionate amounts, 'smearing' white light into a rainbow. This is why, for instance, you might have seen the discussions here about the shorter focal-length violet diodes have relative to red wavelengths through the same lens. (There's been a lot of discussion about this in the thread about 405nm AR-coated lenses, particularly revolving around the need to screw the lens-holder deeper in to the module - blue bends so much more strongly than red that not only does the lens need to be much farther in, the diode itself needs to be pressed into the module farther than 'flush'.) The early PS3 diodes from the KES-400AAA sled had a single 5-pin diode with red, IR, violet, and a photodiode all in the same can. Since these all refract differently, they all have different focal lengths for any specific lens. To compensate for this, there was a lens on a little motor-controlled gantry that slid back and forth to adjust the focus depending on which wavelength was in use at the time. Later iterations have taken 2 forms: completely separate paths through the sled for red/IR and violet (the head actually has 2 lenses on it), or partially-shared paths (where each color takes a different path, but they all pass though a single lens on the head.)


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Old 08-27-2008, 03:11 PM #18
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Default Re: DPSS process

Quote:
Originally Posted by Foobario

On a semi-related note: look at a prism. (If you don't have a prism, the cover of Pink Floyd's Dark Side of the Moon will do.) (If you don't have Pink Floyd's Dark Side of the Moon, may the gods have mercy on your soul.)
*Looks at Dark of the moon vinyl hanging on wall...utters* "true, true" I almost prefer Wish you were here, The Wall or Relics though, but there is no denying the greatness of the dark side! Nice so see a fellow floyd fan!

On a more related not, very informative posts both foobario and pullbangdead!


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Old 09-14-2008, 06:48 AM #19
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Default Re: DPSS process

pullbangdead,

Excellent explanation. very comprehensive and suitably worded.
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Old 09-14-2008, 08:37 AM #20
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Default Re: DPSS process

This makes the light waves come out much like the waves in the 'double-slit' experiment - they 'wrap around' the edges of the slit. They do this in both directions, but since the scratch is much

OMG! I almost forgot about when my freaky red died and started emitting an interference pattern. At first it looked like a normal dead diode then the interference pattern and then the pattern started to warp out.

Can somebody tell me why it did that? :-? My guess is that the bandgap got shorter and then it collapsed.

--hydro15
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Old 09-27-2008, 04:39 AM #21
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Default Re: DPSS process

435 is really more of a deep blue. Look at those blue yard and street lights... mercury vapor lamps. Their primary emission is at 435.8nm.
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Old 09-28-2008, 07:10 PM #22
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Default Re: DPSS process

Quote:
Originally Posted by Murudai
[quote author=Artix link=1219620570/0#6 date=1219640377][quote author=Murudai link=1219620570/0#5 date=1219640151]DPSS gives much better beam specs than diode lasers do.

If you've got a greenie (which is DPSS) and a red (which is almost always a diode) then put them together and compare the beam specs. The greenie should have a tighter beam and better divergence.
At about 100m (meters) my pulsar has a smaller dot then my core...[/quote]

Haven't you seen the Wicked Site? The Pulsar has negative divergence, it actually focuses to a distant point before diverging again.

And also, that's just a matter of focusing it. If you focus it to infinity, you can get tiny divergence. Plus you can get nice divergence on some diode lasers because the beam is so fat. *

Like Frothy said, the beam quality is way better on DPSS systems than diode systems. [/quote]

I bought it from wicked's site!(just over a year ago)

Here is a video that shows my pulsar's negative divergence.
[media]http://ca.youtube.com/watch?v=VoIDNzY9Kgg[/media]
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Old 09-29-2008, 10:07 AM #23
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Default Re: DPSS process

Quote:
Originally Posted by FrothyChimp
435nm is very inefficient (a new SHG type so very low power). They are really novelty items at this point.
Despite its low power it finds some application in biochemistry. Some chemicals absorb this specific wavelength. In simple terms, if you know how much optical power a chemical solution absorb, you can easily calculate the concentration of the chemical responsible for that absorption.
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