Red dot laser apears to be a line laser?
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Red dot laser apears to be a line laser?

by tadpole on Thu Oct 18, 2012 6:04 pm

I recently ordered two red dot lasers, and upon hooking them up, they appear to be line lasers. At about 30ft, they show as a distinctive 1ft line, and not a dot. The packaging had them labled as 650nm dot lasers, as did the order form. Has anyone else noticed this!? As always, I hope this is just a mix up, cause the divergence is really huge if this is supposed to be a dot. Thanks for responding!!! :D

Added info: I also ordered a TTL red dot laser, and just looked at that one, and it is nice dot - not a line like the other two. I'm now 99.99% certain that I got two line lasers, instead of the red dot ones. What should I do now?!
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Re: Red dot laser apears to be a line laser?

by adafruit_support_bill on Fri Oct 19, 2012 6:14 am

Hmmm. For a line laser, the line should be longer than 1' at 30'. But in any case, what you have is not a dot!
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Re: Red dot laser apears to be a line laser?

by adafruit on Fri Oct 19, 2012 9:58 am

In the meantime - if you need to convert a line laser to a dot laser, you might able to just remove the lens in front. it should screw off and you'll be left with a plain dot laser :)
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Re: Red dot laser apears to be a line laser?

by JD3 on Fri Oct 19, 2012 4:02 pm

I also have a dot laser and checked it out to see if I am getting the same result.

I do get a small line, about maybe 3" long at 20' (across the room) and about 1" wide.

I think the focal point is just short, at a couple feet maybe.
At closer distances it is a dot, up to maybe 8'-10'.

EDIT:

I just adjusted the front lens a little back and forth.
I was able to adjust the focal point and get it to be a dot :)
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Re: Red dot laser apears to be a line laser?

by adafruit_support_bill on Sat Oct 20, 2012 6:02 am

@JD3 - thanks for the feedback. Now we know how to fix it!
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Re: Red dot laser apears to be a line laser?

by tadpole on Sat Oct 20, 2012 11:18 am

UPDATE!
I thought maybe it was the lens too, and tried to adjust it to get it more in focus.. I ended up with a much *clearer* line. I've attached a picture of the red dot laser, as opposed to the TTL red dot laser. This is at 8' and the line one is about 1" long.
After reading JD3's coment I messed around with the lens cap more. Removing it wasn't the answer, as this gave me nothing visible if the reflection surface was farther than say 5" away form the laser. It just expanded way too much to see anything. With the lens cap off, and closer than 2", you could see the light, but it was a *bar* across the entire field of view, and not a dot. With further adjustment, as JD3 says, I was able to get a dot! YEAH!
I simply wanted to follow up in case it gives some information to those that know more about lasers than I do. The *sweet spot* in the focus area seems to be very small, and anything above, or below, will still show a line. Thanks a lot to everyone for the help - I do appreciate it!!!
Attachments
IMG_8457.jpg
The two lasers compared BEFORE adjusting the focus.
IMG_8457.jpg (28.59 KiB) Viewed 2942 times
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Re: Red dot laser apears to be a line laser?

by adafruit_support_bill on Sat Oct 20, 2012 11:26 am

Thanks for the follow-up. These are new to the store and we don't have much experience with them yet.
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Re: Red dot laser apears to be a line laser?

by barrychuck on Thu Oct 25, 2012 10:19 am

I'm not sure how you folks missed this but ALL diode lasers have the common problem that they will appear as a line. The diode structure itself will result in the optical resonant cavity having a higher divergence in one plane. Some very expensive diodes under the name circulaser had reduced that factor, but it is dificult to eliminate.

http://www.newport.com/Tutorial-Laser-D ... ntent.aspx

However, divergence of the light emitting from a laser diode is very pronounced with full width half maximum (FWHM) angles of up to 40 degrees in the perpendicular axis (θ⊥) and 10 degrees in the parallel axis (θ||). This divergence results in a rapidly expanding elliptical cone. Gain-guided laser diodes tend to have greater differences between the two angles than index-guided laser diodes. Figure 4 illustrates beam divergence in the theta parallel and theta perpendicular axes.

Figure 4—Schematic representation of an index guided device and its diverging output beam profile.


Thus, the key takeaway is that the only true circular laser beams either come from gas lasers (argon, krypton, HeNe, HeCd, etc..) or, from diode pumped solid state (DPSS) lasers because they have larger optical cavities and thus limit the divergence in all planes resulting in a circular beam. While the effect is minimized at the focal point of the lense, it's should be obvious it's impossible for perfect focus if one axis is ~40 degrees divergent and the other is ~10 degrees.


Also, this would mean that blue diode lasers such as those from Blueray drives exhibit the same divergence as the typical red diode laser. Even the infrared diode laser does this in a DPSS, but as stated, the DPSS also has an external cavity and thus you never directly see the divergence from the diode.
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