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Fast and slow axis

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Hello, so, I was finetuning the infinity focus of my Gatling, and I noticed that the axis that starts out wider is actually the less divergent axis, and the initially smaller axis is actually the more divergent axis. Is this normal? I can't wrap my head around why this would happen, I believed that because the initially wider axis was so because it has a higher divergence. How does a lens reverse this? I assumed that the lens reduces the divergence of both axes equally, so eventually the narrower axis would be perfectly focused while the wider axis would still diverge. Sorry if this is a stupid question.
 



Encap

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Last edited:
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Thanks, but my question is about the fast axis turning into the slow axis after passing through the lens. This article talks about the raw diode output. I measured it, at point-blank the spot is about 2x4mm, and at about 7m it's about 17x7mm (the 2mm axis expands to 17mm, and the 4mm axis to 7mm). When I focus the laser to a point however, the axes don't swap.
 
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See: https://www.rp-photonics.com/broad_area_laser_diodes.html

and https://blog.rpmclasers.com/laser-diode-fundamentals-beam-properties

If you want better beam quality than is possible with diode lasers you need a different type, a DPSS laser which produces Gaussian round beam and is much better quality than any diode. Example : JetLasers 532nm PL-E and PL-E Mini
It appears you edited your reply. To be clear, I am perfectly fine with the divergence on the multimode diode. It's just that the fast axis becoming the slow axis after passing through the lens is mind-boggling to me. The second article also talks only about the raw diode output.
 

ArcticDude

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Hey, pls do not double post..
You can edit your previous post to combine em..
 
Joined
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Hey, pls do not double post..
You can edit your previous post to combine em..
Sorry, I'll try not to do that. I just messed around with the lens a bit more. What seems to be happening is that when the laser isn't focused to infinity the fast and slow axes are the same as before passing through the lens, but when it gets close to infinity, at some point, only the fast axis gets tighter, and eventually becomes the slow axis. I don't see any focal points in the beam in either axis. I don't understand why this happens. I understand why there is a fast and slow axis in the raw diode output, but not what happens after passing through a lens.
 

ArcticDude

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Sorry, I'll try not to do that. I just messed around with the lens a bit more. What seems to be happening is that when the laser isn't focused to infinity the fast and slow axes are the same as before passing through the lens, but when it gets close to infinity, at some point, only the fast axis gets tighter, and eventually becomes the slow axis. I don't see any focal points in the beam in either axis. I don't understand why this happens. I understand why there is a fast and slow axis in the raw diode output, but not what happens after passing through a lens.
I hope that you'll understand that inputting more post ain't gonna give any more answers, only frustrated members
 
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I hope that you'll understand that inputting more post ain't gonna give any more answers, only frustrated members
Yes, I understood. By the way, doesn't the lowest possible divergence reduce proportionally to the apertude width? I mean, that's why beam expanders exist. That might explain why the initially fast axis becomes the slow one, because the aperture diameter is higher.
 

Cyparagon

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All else equal, beam divergence is inversely proportional to beam diameter. Once you understand that, everything else is predictable.

What's really going to bake your noodle is finding out the raw diode output without a collimating lens is yet again flipped in this regard..
 
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All else equal, beam divergence is inversely proportional to beam diameter. Once you understand that, everything else is predictable.

What's really going to bake your noodle is finding out the raw diode output without a collimating lens is yet again flipped in this regard..
Already found that out, about the raw diode output, from what Encap linked. What I still don't understand is why, when approaching infinity, the beam keeps getting thinner on one axis but the other one stays the same, despite the focus being changed.
 




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