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New purple laser isn't a dot

xlinker

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I just got a new laser from eBay which is a 405nm laser. When I first turn it on it wasn't a dot. I had to focus it for it to at least look like a dot but still doesn't look like a solid round dot. Are purple laser supposed to do this? Is something wrong?

Without any focusing it look like a rectangular, oval ish shape.

https://imgur.com/gallery/a34Ly
 

Merpie101

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thats because the diode is multimode, so its not going to ever really be a round dot. going to be closer to a rectangle.
 

ZRaffleticket

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That output looks fairly normal to me?

By "it doesnt look normal" does it look fuzzy at a distance, kind of like how a camera has something out of focus?
 

Alaskan

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Even single mode laser diodes aren't a round dot, not the ones I have had, not yet.
 

BobMc

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It sorta looks like the laser isn't focused completely. Have you tried to focus it more?
 

ZRaffleticket

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^ in the pic he provided he said it was unfocused on purpose.

I think it's just that our eyes aren't built to look at focused 405nm light at a distance. I think he's observing a halo around the dot at a distance, where up close I bet he won't see it.

Edit: it's similar to how red text on a blue background / vice versa is not very focused...
 
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diachi

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Looks perfectly normal for a diode laser, at least as best I can tell from the image.

Hap, it's likely single mode but even then, you still don't get a perfectly round beam.
 

ZRaffleticket

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Also to note, multimode doesn't always equate to a square or rectangular beam. The 120mW PLP520B diodes are multimode but still have an arguably round beam.
 

BobMc

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Didn't catch it wasn't fully focused. :tinfoil:
Sure would like to see what the dot would look like when it was fully focused as tightly as possibile. :thinking:
 
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Alaskan

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Also to note, multimode doesn't always equate to a square or rectangular beam. The 120mW PLP520B diodes are multimode but still have an arguably round beam.
Cool, didn't know that, now I have to go look. My PL520 single modes put out rectangular.
 

Nutball

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My multi mode pocket laser looks like that, but most emitters are not circular, so they will not make a truly round dot correct? And, most cheap lasers we see are not designed to be focusable, so we are used to seeing a tiny dot that is actually not perfectly round when you enhance it's shape by widening the beam.

I have a question: I always thought my multi mode laser used a single mode diode, but had a regulator that controlled input current for the 3 brightness setings, and timed the on/off power to cause strobe effects. So what is this talk about multi mode diodes, and why are they beneficial over controlling the input of a single mode diode? I have seen in an avatar or two diodes with more than one gold wire attached, so I've assumed those are either multi mode, or some sort of high power diode.
 

ZRaffleticket

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Multimode isn't in reference to the driver settings (high power, medium power, low power, pulsed, etc), but rather to the emitters in the diodes themselves. Single mode diodes have a small, round(er) emitter, where multimode diodes have larger emitters... The geometry of the emitter has a lot to do with giving the square/rectangular beam the 1W+ diodes have

And yes -- the spots are only truly round with VECSEL/VCSEL diodes, which are primarily used for very precise DPSS/OPSL requirements due to their near perfect circular nature. Our single mode diodes usually have an oval shaped dot, however it's round enough for us to call it round.
 
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diachi

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My multi mode pocket laser looks like that, but most emitters are not circular, so they will not make a truly round dot correct? And, most cheap lasers we see are not designed to be focusable, so we are used to seeing a tiny dot that is actually not perfectly round when you enhance it's shape by widening the beam.

Correct, the resonator in a laser diode is square/rectangular, so you'll don't get a perfectly round beam, even from a single mode emitter.

I have a question: I always thought my multi mode laser used a single mode diode, but had a regulator that controlled input current for the 3 brightness setings, and timed the on/off power to cause strobe effects. So what is this talk about multi mode diodes, and why are they beneficial over controlling the input of a single mode diode? I have seen in an avatar or two diodes with more than one gold wire attached, so I've assumed those are either multi mode, or some sort of high power diode.

You're talking about something else - that's power/operation modes. Those happen at the driver. We're talking about transverse modes.

Multi-mode diodes operate on higher order transverse modes. More power requires a larger emitter/resonator, which in turn results in multiple transverse modes being present in the resonator.

The bond wires don't really have anything to do with it, there's just more of them in order to handle the increased current required for a larger emitter.

https://www.rp-photonics.com/modes.html
See answers in red. :)

Multimode isn't in reference to the driver settings (high power, medium power, low power, pulsed, etc), but rather to the emitters in the diodes themselves. Single mode diodes have a single emitter, where multimode diodes have multiple emitters (usually in a line -- giving the square/rectangular beam the 1W+ diodes have).

Incorrect, multi-emitter diodes have multiple emitters. Those emitters themselves may be multi-mode. The multi-mode diodes we use in pointers are all single emitter diodes (with the exception of those dual-beam/tri-beam reds that were going around). The square/rectangular beam is a result of the resonator geometry (i.e. rectangular), not the result of multiple emitters.


And yes -- the spots are only truly round with VECSEL/VCSEL diodes, which are primarily used for very precise DPSS/OPSL requirements due to their near perfect circular nature. Our single mode diodes usually have an oval shaped dot, however it's round enough for us to call it round.
See answer in red. ;)

This is what a multiple emitter diode looks like (or at least one example of what they look like):



You can sort of sea the individual emitters, there's 19 of them in a row. The image is a little saturated, there's a better one somewhere where you can see the emitters more clearly...

Edit: Found a better example from one of my own systems. You can see clearly each individual emitter.

 
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