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beam expander logic

justinjja

justinjja

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1st, a beam expander allows a laser beam to diverge less, at the expence of a larger beam up close
^is that correct?

If that is correct, would having a collimating lens designed to be say, 1 inch away from an axix module work in the same way?
 
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Cyparagon

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It certainly does. I got carried away a couple weeks ago. :eek:

635/638nm; 3mW pointer/300mW mitsubishi diode; 4mm/70mm beam:

SAM_2953.jpg


SAM_2952.jpg
 
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Sigurthr

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That first picture brings back memories of TRON... the original, not the sequel.
 
S

steve001

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justinjja said:
1st, a beam expander allows a laser beam to diverge less, at the expence of a larger beam up close
^is that correct?

If that is correct, would having a collimating lens designed to be say, 1 inch away from an axix module work in the same way?
Click to expand...

That wouldn't work probably if you want a highly collimated beam of low divergence.
You have to consider the expanding lens. It's very important.
Play around with this applet gausian beam rev 2.0.xls
In the first 2 boxes at the top input your lasers beam diameter and the beams divergence.
In the boxes below input a negative focal lenght ( ex. -6) and the box below that a positive focal length. This applet has a graphical result presentation also
 
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Cyparagon

Cyparagon

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steve001 said:
That wouldn't work probably if you want a highly collimated beam of low divergence.
You have to consider the expanding lens. It's very important.
Click to expand...

Nope. The beam is already expanding when it leaves the diode facet. So:

1) beam is expanding from diode
2) beam is collimated with lens
3) beam is expanded with lens
4) beam is collimated with lens

Steps 2 and 3 are not necessary.
 
rhd

rhd

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What's the logic / math involved in taking a divergence figure for both the fast & slow axis, and using them to calculate the focal length and distance from LD for a single collimating lens?
 
rhd

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ElektroFreak said:
Here's yer maths: Focusing and Collimating

(No I'm not versed in the workings thereof, I just know this to be a very good resource..)
Click to expand...

I've read that before. It's a neat resource, but diode specs are generally given in degrees, for each axis. Plus the emitter size is often unknown. It strikes me that there should be a way to simply use the divergence angles of the diode, the lens's focal length, and calculate the rest.
 
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ElektroFreak

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rhd said:
I've read that before. It's a neat resource, but diode specs are generally given in degrees, for each axis. Plus the emitter size is often unknown. It strikes me that there should be a way to simply use the divergence angles of the diode, the lens's focal length, and calculate the rest.
Click to expand...

The emitter size is often (but not always) unknown to us as hobbyists, but if we were in the profession we would have looked all that info up before selecting a diode for whatever application we're trying for. Being hobbyists leaves us at a bit of a disadvantage sometimes.. but speaking for myself I love the trial and error approach. Tends to make for some interesting learning experiences.

As far as degrees are concerned you can convert from radians by multiplying the number of degrees by 180°/(Pi)
 
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BShanahan14rulz

BShanahan14rulz

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I think RHD is trying to ask which single variable to use, the fast axis or slow axis divergence value. Or even how to use both of the two differing divergences to create an equivalent divergence value for calculations.
 
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ElektroFreak

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With laser diodes you're not going to be able to use both divergence figures simultaneously because of the astigmatic beam, but I think you can use the data for a single axis for collimating with normal concave or convex lenses like we do with our Aixiz modules etc.. In order to balance the axes to get a symmetrical beam you'd need to collimate the fast axis first, making the optical train more complex. These problems are inherent to all laser diodes, and they account for one of the biggest reasons why nice, round TEM00 beams are so desirable.
 
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S

steve001

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Cyparagon said:
Nope. The beam is already expanding when it leaves the diode facet. So:

1) beam is expanding from diode
2) beam is collimated with lens
3) beam is expanded with lens
4) beam is collimated with lens

Steps 2 and 3 are not necessary.
Click to expand...

Yes, all laser beams expand. And you can collimate a beam with just one positive lens, which has to be placed at or close to the lenses focal length from the diode. However, all beam expanders use at least two lenses I think for the reason that lower divergence can be achieved over using one lens. Using one lens I think requires rather large lenses to be used to catch all of the beam if you don't want to truncate the sides of the beam.
Assuming the lens you used is placed at the focal length distance from the diode it appears the beam is expanding greatly has it exits the front surface. Though collimated the beam has high divergence which is acceptable if that is your goal, but usually it's a beam with low divergence that's desired.

I don't know what the OP is trying to accomplish maybe they'll explain in detail..
 
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