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Single mode and multi mode

Sideburn

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I’ve been curious about the differences between single mode lasers and multimode but a search brings up technical details mostly regarding fiber optics.

i was wondering more about laser pointers and if you can easily tell if you have a single or multimode. I assume my < 100mw greens must be single mode but what about the Chinese blue 450nm 1W laser? Is that one multimode?

and what exactly is multimode? Two different (and in color) beams? For more power?

can you tell by the quality of the beam?

and why does the focus on the 1W laser turn into a blurry rectangle vs a circle (dot) ? Is this related?

lots of questions I know, and thanks in advance for any info.
 



Sideburn

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Thank you! I will read up on the first link.
I already found that second link.
Can you tell me what is causing the rectangular dot / focus on the 1W blue laser?
 
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hakzaw1

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A rectangle that might be 'normal'
not all diodes make a dot-- some lenses help a bit. is that one a 9mm can or 5.6mm
afaik more of the smaller diodes make a dot-- most of the 9mm do not.
You may lose some mW when the ends are clipped .

post a pic and someone will chime in..
wish I could help more.... len
 

Sideburn

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It’s one of those Chinese lasers on eBay if you search “1W blue laser”

I can post a pic of the “dot”. It’s a round dot unless you take it out of focus then the dot gets wide and turns into a rectangle as shown in this photo. The red arrow is pointing at what the dot looks like when out of focus.

3D07BB6D-7AEE-4C13-BB3D-8C126D9CF3E1.jpeg
 

Encap

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That is normal for high power diode lasers due to the physical nature, size and shape, of the diode emitter and the fast and slow axis divergence characteristics being different and lack of expensive pairs of cylindrical lenses to correct as much as possible for the differences in low cost and quality mass produced laser pointers. Is not worth doing as it matters little if at all for the purposes laser pointers are used for + people want and buy cheap/low cost laser that make a beam and dot, they have no need, use for, or understanding of top quality lab grade lasers with near perfect Gaussian beam quality.

"edge-emitting laser diodes where the emitting region at the front facet has the shape of a broad stripe (see Figure 1), with dimensions of e.g. 1 μm × 100 μm. Due to the asymmetry of the emitter, the beam properties are also completely different for the two directions:
  • In the vertical (short) direction, the height (e.g. 1 μm) is small enough to obtain single-mode guidance and thus an essentially diffraction-limited beam quality with an M2 factor only slightly above 1. Because of the small aperture size, the beam divergence in this direction is relatively high, with a beam divergence half-angle of e.g. 370 mrad, corresponding to an FWHM angular range of 25°. Due to that fast divergence, this is called the fast axis direction.
  • In the long direction (slow axis direction), the stripe width may be e.g. 50, 100, 200 μm, or even larger, so that the light is distributed over many spatial modes in this direction. As a result, the beam divergence is much larger than for a diffraction-limited beam with that size, although still significantly smaller than for the fast axis direction. (Typical values are around 5–10° FWHM.) The beam quality in terms of focusability is reduced; M2 might be of the order of e.g. 20 for a 100-μm stripe. Furthermore, the beam profile may be multi-peaked in the horizontal direction, and the shape of the intensity pattern may depend on the injection current." from: https://www.rp-photonics.com/broad_area_laser_diodes.html
You can see the effect of different lenses on the dot shape of same diode on DTR's page if you scroll down: https://sites.google.com/site/dtrslasershop/home/glass-lenses

If you want or need better quality beam and dot you need to go with DPSS lasers or make a laser pointer yourself that incorporates that employs multiple expensive lenses to correct the divergence differences
 
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Sideburn

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So that laser is a single mode? Are all of the low cost 1mw - 1W laser pointers single mode and DPSS?
 

Encap

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So that laser is a single mode? Are all of the low cost 1mw - 1W laser pointers single mode and DPSS?
Same question over and over?
Keep reading upon what's what until you begin comprehend what you read.

No, it is not a single mode it is just a mass produced low cost low quality multimode laser pointer.
No to the second question as well.

If you want to see a range of good relatively low priced DPSS lasers -none of them pointers--all of the more expensive than any pointers- see: http://www.cnilaser.com/
You can check prices from a CNI Canadian distributor here: https://www.ultralasers.com/lasers-series.php?cat=21
 
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dreaming

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wow what a D!ck, but thanks for sort of answering my questions. Uhm no I was looking for a place that might be able to answer some questions As I learn more about how lasers work. Are those your sites where you sell your over priced lasers? Who’s the troll now. Thanks anyway. Mine are fine for now.
Tech forums like this one are not generally receptive to questions that have been answered before.

If you want a more laid back place to ask this stuff, try doing a search for reddit lasers.

If you mind people suggesting that your questions are stupid, you might want to leave the internet, permanently, because internet anonymity breeds contempt.
 

Lifetime17

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Hi ,
As Encap stated go to the DTR site and he lists what diodes are single and Multi Mode diodes. From there you can get the diode numbers. if you want to buy from some Chinese vendor, But I would buy from DTR that way you know what your getting. The Chinese vendors some not all have a way of BSing what they sell. And DTR is here in the states and can set you up with the correct diode / driver / lens package.
Rich:)
 

Sideburn

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Tech forums like this one are not generally receptive to questions that have been answered before.

If you want a more laid back place to ask this stuff, try doing a search for reddit lasers.

If you mind people suggesting that your questions are stupid, you might want to leave the internet, permanently, because internet anonymity breeds contempt.

my bad, I didn’t know this was an expert laser pointer people only forum. I’ll take my “stupid” questions elsewhere or go to laser school first.
 

Sideburn

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Hi ,
As Encap stated go to the DTR site and he lists what diodes are single and Multi Mode diodes. From there you can get the diode numbers. if you want to buy from some Chinese vendor, But I would buy from DTR that way you know what your getting. The Chinese vendors some not all have a way of BSing what they sell. And DTR is here in the states and can set you up with the correct diode / driver / lens package.
Rich:)

thanks yeah I realize the cheap Chinese eBay lasers are marketed as underpowered “cat toys” which is pretty scary. I’m not ready for high end yet I’m just now starting to learn more about how they work, experimenting, learning about dangers/safety etc. I will check out the links and DTR though and my next round will be quality. Then I’ll mount a 10W laser To my cabinet and point it at the fireplace so I can say “Alexa light the fireplace with my laserrr” 😂
 

Lifetime17

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my bad, I didn’t know this was an expert laser pointer people only forum. I’ll take my “stupid” questions elsewhere or go to laser school first.
Hi Sideburn,
Go to the DTR laser store site he has many single mode diodes. Single modes are of lower power but produce a nice dot. To were as Multi mode diodes are of higher power and produce a rectangular spot. The Gattling you have has a Multi mode in it thats why the dot is rectangular . If your thinking about changing the diode in your laser dont they are glued together very well so you cant fix them.
If you want a custom laser host or a flashlight modded host I make them. All you would need to do is buy the diode of your choice with driver prewired and solder two wires to the two wires in the host . Hope this helps you some what. I wouldn't say stupid its just your new to the game but in time im sure you will learn different things. Dont be offended everyone is different in their own ways hang in there.
Rich:)
 

Sideburn

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Hi Sideburn,
Go to the DTR laser store site he has many single mode diodes. Single modes are of lower power but produce a nice dot. To were as Multi mode diodes are of higher power and produce a rectangular spot. The Gattling you have has a Multi mode in it thats why the dot is rectangular . If your thinking about changing the diode in your laser dont they are glued together very well so you cant fix them.
If you want a custom laser host or a flashlight modded host I make them. All you would need to do is buy the diode of your choice with driver prewired and solder two wires to the two wires in the host . Hope this helps you some what. I wouldn't say stupid its just your new to the game but in time im sure you will learn different things. Dont be offended everyone is different in their own ways hang in there.
Rich:)

Thank you.. all good... I don't mean to be harsh either, just got kind rubbed the wrong way with the whole "silly question" / "troll" thing...
I am good with the lasers I have right now I was just curious about the differences between single mode and multimode and was looking for a more laymen's term answer about the basics and what is in these low end laser pointers.. I'm all for technical details, I am a hardware and software engineer so I get it... but if someone asked me what a bitshift was I wouldn't call them a troll and that it was a sllly question 😂 I'm just a total newb when it comes to lasers right now and want to start with the basics before diving into all the stuff thats going over my head at this point. I'm not easily offended either so no worries...
 
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CurtisOliver

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I recommend this site for your reference.

 




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