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ArcticMyst Security by Avery

Raw output of packaged diode shows "moire" patterns

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Nov 9, 2012
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I noticed the raw output of a laser diode I have shows these 'moire' like patterns or shadows. What wold you suggest is the problem or the cause of the issue?

I did wipe the window a bit to make sure theres no dust particles for the beam to diffract from. Is it a bad diode? Is it dust I am not cleaning away? Does it matter too much once I focus and collimate the beam to create a pointer?

Also, how do you create a true TEM00 output beam using laser diodes with elliptical output?
 





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Aug 3, 2011
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What diode? What driver? Pictures? Were gonna need a lot more info to answer this one.\

I would suggest NEVER touching the diode window. You may crack the window, and its not very likely you will clean anything off of it. If its even dirty at all that is.
 
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those Moire-esque patterns are interference patterns from light diffracting around particles or other imperfections on the window, or sometimes even on the facet. You won't notice after you use a lens to further collimate your laser.

I've had decent luck with using 91% isopropyl from the grocery and some 100% cotton swabs. The swabs are good for a single wipe in one direction. You don't want to be rubbing whatever the swab picks up on the first pass into the glass on the second pass.

True TEM00 mode, meaning a single classical gaussian mode that is circular when viewed orthogonally? You would need further beam correcting optics to get close to approximating this. I haven't gotten to play with optics as much as I'd like, but it looks like you could use a cylindrical lens or some prisms and obtain somewhat usable results.
 
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Its a tiny 3.3mm red diode had laying around.

Yeah figured it was diffraction. I think its the surface window not very clean. The diode is kinda small but wiped it down somemoret and the pattern changed (to streaks). Probably some organic residue. Might try cleaning with IPA later.

Actually this is interesting. Is the elliptical output from these diodes technically still considered TEM00 anyway, but astigmatic? Or is TEM00 strictly for rotational symmetry with gaussian profile?
 
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The elliptical output is because the diode has a fast and slow axis, the modes refer to the power distribution in the cavity, ie, "higher" TEM modes use more of the cavity, thus you can sometimes obtain higher powers with strange TEM modes, if you don't mine the odd shape. Looking at the raw diode output is misleading as you won't see any of those artifacts once it's collimated. Once collimated, you'll be able to see which TEM mode it's running easily, which for diodes is almost always TEM00 unless you're running it out of it's optimal temp range, or overdriving it (Which again effects the die temperature). I have seen diodes mode shift, but under normal operation it's rare unless it has imperfections in the cavity/facets from manufacturing. Much, much more common in DPSS.
 




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