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Olozhika
Wicked Lasers God
Joined: 20 Aug 2006
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 Posted: 2/18/07, 12:06 PM Post subject: Beam Diver |
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What is divergence?
All lasers, no matter what type have beams which grow bigger over distance.
A diverging beam will look something like this:
Now, at this point I will just say that divergence is not due to bad workmanship or lack of quality, it is a result of the properties of light itself and therefore is completely unavoidable.
Are there any laser that does not diverge?
No, all lasers diverge no matter what type.
What lasers do we use?
The lasers that we generally use are “diode based”, diode lasers have numerous advantages over all other types of lasers as well as an equal amount of disadvantages.
- Diode lasers are cheaper and easier to mass produce.
- Diode lasers use very little power and can therefore be made portable.
The above is what has made diode lasers affordable and can now be seen in many people’s homes. However I also mentioned that diode lasers have disadvantages, these include:
- Generaly not the best beam qualities when compared to HeNe Lasers.
- Diodes are available in a variety of colors but unfortunately anything other than IR and Red at the moment is increadably expensive.
- Some diodes are very high quality but are not widely used due to their price tag.
Why can’t a small beam have good divergence?
The answer to this question is a very complex scientific one which will be far too involved for most people so rather than explain in detail I will simply tell you in an abbreviated, simplified form.
What we perceive as light is part of the electromagnetic spectrum, between 390nm to 800nm.
Here is a graph of the electromagnetic (EM) spectrum.
Notice some of the key terms in this graph, wavelength, nanometers (nm).
Light is very much like a wave, which is why we call it wavelength.
We measure wavelength in nanometers which is frequently abbreviated to nm.
One nanometer is equivalent to 1 billionth of a meter so it is very small.
As you can see in the diagram above, the space between each of these “waves” is measured in nm, red lasers would have their “waves” spaced every 650nm whereas a green laser would have its “waves” spaced every 532nm.
Our eyes are designed to perceive these wavelengths as colors which allows us to see.
In our everyday life green light is less abundant than red and blue which makes our eyes more sensitive to the green portion of the spectrum. This is the reason why green lasers seem to be far brighter than other colors.
Now that we understand some of the basic properties of light we can more back onto our original question of “Why can’t a small beam have good divergence?”
According to scientists who have studied this in great detail, they state that beam diameter and divergence are relative to each other. As a general guide a laser can be collimated to Y, where 1 is divided by the initial beam diameter.
To put this into perspective, if we had a laser with an initial beam diameter of 2 millimeters then we can work out the possible divergence for this laser with the following calculation:
1 ÷ X = Y
X = Initial beam diameter
Y = Possible Divergence
So, if we do this calculation and substitute X for 2 as mentioned above our answer (Y) should be 0.5 mRad
This figure we have got is the aproximate possible divergence that can be achieved with the standard optics.
The Evolution series lasers have an initial beam diameter of approximately 0.8 mm which if we perform the above calculation will give us a theoretical aproximate possible divergence of 1.25mRads.
The Executives on the other hand have an initial beam diameter of approximately 1.8-2.5mm which gives us an estimated divergence of between 0.4 and 0.55 mRads.
Wait a minute what are mRads? We will go on to answer this in the next section.
What are mRads?
mRad is short for milliradians, which is a scale of measurement which we use to measure divergence of a laser.
A divergence of 1 mRad would be equal to the beam expanding 1mm for every 1 meter it travels.
This is not the exact definition of a milliradian though we generaly use this 1mm to a meter for lasers, please remember that if you are trying to calculate the divergence of something which is measured in degrees then you will see that your calculations will be off.
With lasers, since the divergence is so small the amount of error we would get from this calculation is so miniscule that it is insignificant.
So, to put this into perspective, if we got our Evolution laser and shone it up into the sky, after 1 kilometer our beam would be 1.25 meters in diameter.
Alternatively if we did this with our Executive laser, we would expect our lasers beam to be 0.502 meters in diameter.
What is my divergence?
To measure this is quite simple. First off some tools that you will need:
- A measure (ruler, tape measure)
- A matt black surface, such as a black t-shirt.
- A way to keep your laser switched on, cable tie or a friend will do.
Right, find a nice open space and measure the distance between point A and point B.
Point A is where your laser will be set up and switched on, point B is where you will be measuring it.
A – Start point
B – Initial Beam Diameter
C – Beam Diameter at 10 meters
D – Beam Diameter at 25 meters
The further away you can measure your laser the more accurate your results will be.
All you have to do is measure how wide your beam is at a given distance.
We would then imput these figures into a calculator like this one:
Pseudonomen137’s Calculator
This will give you your lasers divergence in mRads.
This concludes the introduction to laser divergence.
_________________
----------------------------------------------------------------------
 Divergence: <0.49mRad
 Divergence: <1.3mRad
 x 2
----------------------------------------------------------------------
View my website with up to date news, reviews and tutorials!
Last edited by Olozhika on 2/18/07, 1:51 PM; edited 4 times in total |
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pseudonomen137
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| Posted: 2/18/07, 12:21 PM Post subject: |
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Wow you really put a lot of time into that didn't you? Good job, and if I were a mod I'd sticky it. One minor and unimportant detail to nitpick though: Most of the times we say 1 mRad is 1 mm expansion over 1m. This is not the true definition of it, and when you get into very high amounts (many degrees) of divergence, you start to see a bit of a margin of error. It does work well as an approximation for low divergences because the trig function behind it follows the y=x line very tightly for small angles, but mRads are really a measure of the degree of the angle the light spreads out by. (For beams of a few mRad, I think the margin of error is some nearly-infinitesimal number, so it doesn't really matter anyway)
EDIT: Oh, and I think the equation you were touching on is: Minimum Divergence is proportional to Wavelength / Beam Diameter. What you say about the 1/x isn't quite right because you're forgetting the equation is about proportionality, and isn't meant to be used with these strict units (at least as I understand it). What the question helps you with is that if you have a well collimated beam at 1mm at aperture and 2mRad divergence, you could trade that off with a different lens for say... 0.5mm at aperture and 4mRad, or 2mm at aperture at 1mRad, etc.
EDIT2: And also, I think you make some unwarranted generalizations about diodes. Some have extremely good beam quality (check out VSCELs), and they are available in many diff colors/wavelengths if you are willing to front the production costs. I'm also not sure where the "less reliablility" comment came from.
All in all, still a good guide, but with a few kinks to be worked out if you want to edit it.
Last edited by pseudonomen137 on 2/18/07, 12:34 PM; edited 3 times in total |
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Orr3
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Olozhika
Wicked Lasers God
Joined: 20 Aug 2006
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| Posted: 2/18/07, 1:04 PM Post subject: |
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| pseudonomen137 wrote: |
Wow you really put a lot of time into that didn't you? Good job, and if I were a mod I'd sticky it. One minor and unimportant detail to nitpick though: Most of the times we say 1 mRad is 1 mm expansion over 1m. This is not the true definition of it, and when you get into very high amounts (many degrees) of divergence, you start to see a bit of a margin of error. It does work well as an approximation for low divergences because the trig function behind it follows the y=x line very tightly for small angles, but mRads are really a measure of the degree of the angle the light spreads out by. (For beams of a few mRad, I think the margin of error is some nearly-infinitesimal number, so it doesn't really matter anyway)
EDIT: Oh, and I think the equation you were touching on is: Minimum Divergence is proportional to Wavelength / Beam Diameter. What you say about the 1/x isn't quite right because you're forgetting the equation is about proportionality, and isn't meant to be used with these strict units (at least as I understand it). What the question helps you with is that if you have a well collimated beam at 1mm at aperture and 2mRad divergence, you could trade that off with a different lens for say... 0.5mm at aperture and 4mRad, or 2mm at aperture at 1mRad, etc.
EDIT2: And also, I think you make some unwarranted generalizations about diodes. Some have extremely good beam quality (check out VSCELs), and they are available in many diff colors/wavelengths if you are willing to front the production costs. I'm also not sure where the "less reliablility" comment came from.
All in all, still a good guide, but with a few kinks to be worked out if you want to edit it. |
Thank you Pseudo, I was actualy counting on the more knowledgeable members here to look through it and error check it. I will go and make those edits now as this is supposed to be reliable, factual information.
Also if anybody else has anything to add please post.
_________________
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Divergence: <0.49mRad
Divergence: <1.3mRad
x 2
----------------------------------------------------------------------
View my website with up to date news, reviews and tutorials! |
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FuSion
Wicked Lasers Master
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| Posted: 2/18/07, 1:14 PM Post subject: |
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I would sticky this, but where? Here or Help / Safety? What do you guys think...
~FuSion
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Olozhika
Wicked Lasers God
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| Posted: 2/18/07, 7:36 PM Post subject: |
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I have fixed those points that you mentioned Pseudonomen137.
BTW, do you mind me linking to your website's calculator?
_________________
----------------------------------------------------------------------
Divergence: <0.49mRad
Divergence: <1.3mRad
x 2
----------------------------------------------------------------------
View my website with up to date news, reviews and tutorials! |
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nes_ds
Wicked Lasers God
Joined: 02 Nov 2006
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| Posted: 2/18/07, 7:56 PM Post subject: |
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| FuSion wrote: |
I would sticky this, but where? Here or Help / Safety? What do you guys think...
~FuSion |
Here, because this is about general laser information 
Also, thanks for the info!  |
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Athoul
Wicked Lasers God
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Steve0000
Wicked Lasers Master
Joined: 27 Aug 2005
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| Posted: 2/19/07, 10:18 AM Post subject: |
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There is something else that you forgot to talk about that is the Rayleigh Range. The Rayleigh Range starts at the point where the beam exits the laser and is know as the Near Field. Within that field the laser beam does not expand linearly with distance, it does expand though but at a greatly reduced rate. Divergence is also determined by the initial waist diameter and divergence before the beam leaves the laser and passes through the lenses. The focal lengths of the lenses themselves necessarily play a role in the final beam divergence.
From the Melles Griot site :
| Quote: |
In a Gaussian beam, the distance from the beam waist that the beam waist expands by a factor of the square root of 2 (21/2). The value for the Rayleigh range is given by
zR = pw02/l
where w0 is the beam waist and l is the wavelength of the light.
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That's 1.41 times its initial diameter
With an optic system designed with low divergence in mind the Rayleigh Range can be many meters to tens of meters or more. Related topics are NEAR and Far Fields |
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fnugget
Elite Laser
Joined: 21 Jul 2005
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| Posted: 9/11/07, 1:39 PM Post subject: note |
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another note, beam size is generally determined by the point at which beam intensity drops by 1/e of the maximum (center).
i.e. if you're going to measure, have a consistant measure based on a % of maximum. |
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