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# correcting a NDG7475

#### CDBEAM777

##### Well-known member
Well....I cannot resist.....Our C-Lenses Multiply the Slow Axis.......so that we more closely approximate X Axis=Y Axis at the Far Field....that is why we see the terms 3X, and 6X. The C-Lenses reduce the X/Y astigmatism.....by multiplying ( Read expanding ) the Slow Axis !!!

Now...one would think..." Hmmmm….Slow Axis....Fast Axis.....Hmmmm...This means Slow Axis =Slow Divergence....and Fast Axis = Fast Divergence....and common sense would suggest ...hmmmm...we want to tame that darn Fast Axis,,,,,and reduce that Fast Divergence....to get away from that bar output....Right ????....Well NO....WRONG....we need to multiply the Slow axis.....so that is ALMOST matches the Fast axis....and THAT is what we do with
C-Lenses.

I know it sound's stupid....BUT....THAT is what is going on....when we correct the beam !!!

Optics are crazy hey ???? It has taken me a long time...to believe this explanation….but....it is true !!

CDBEAM

#### logsquared

##### Active member
Yes, thats the one. Thanks for posting.

In general, not necessarily your case RC, I guess its easy to see how it can get confusing. If you just look at a module or pointer and visually see that one axis expands more (faster) intuitively it would seem that is the fast axis. One just has to look behind the lens to see what is really happening.

#### RedCowboy

##### Well-known member
Hold on one second here, CDBeam just said we are working with the slow axis
Well....I cannot resist.....Our C-Lenses Multiply the Slow Axis.......so that we more closely approximate X Axis=Y Axis at the Far Field....that is why we see the terms 3X, and 6X. The C-Lenses reduce the X/Y astigmatism.....by multiplying ( Read expanding ) the Slow Axis !!!

Now...one would think..." Hmmmm….Slow Axis....Fast Axis.....Hmmmm...This means Slow Axis =Slow Divergence....and Fast Axis = Fast Divergence....and common sense would suggest ...hmmmm...we want to tame that darn Fast Axis,,,,,and reduce that Fast Divergence....to get away from that bar output....Right ????....Well NO....WRONG....we need to multiply the Slow axis.....so that is ALMOST matches the Fast axis....and THAT is what we do with
C-Lenses.

I know it sound's stupid....BUT....THAT is what is going on....when we correct the beam !!!

Optics are crazy hey ???? It has taken me a long time...to believe this explanation….but....it is true !!

CDBEAM
When I use a G2 lens on a nubm44 I get a line/bar at 15 feet that's 1.75 - 2 inches long and it's a horizontal line, so my c-lens pair sets between the G2 and my target, I don't expand the slowly expanding axis but I reduce the rapidly expanding axis by expanding it near the G2 with the concave cyl then reigning in that expanded beam with the convex cyl
I trade a wider starting beam for a tighter finishing beam.
My c-lens pair is employed upon the more rapidly diverging axis.
If I expander the slower expanding axis then my bar/spot would be 2 inches by 2 inches, but that's not what happens, I end up with a 7 x 2mm bar/spot at my target.

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#### logsquared

##### Active member
CDbeam is correct. The divergence is opposite the raw diode after the colimation lens. This is because the lens is imaging the emitter far field. The raw diode divergence doesn't have anything to do with the after lens divergence. The Raw diode divergence only dictates the beam dimensions at aperture (the lens). Its the dimension of the emitter and the power of the lens (fl) that determine the after lens divergence.

#### RedCowboy

##### Well-known member
It must be in how it's said then.
I do reduce the spread of my beam between the output of the G2 and it hitting my target.
The 2 inch long by 2mm tall spot becomes a 7mm long by 2mm tall spot.
This is happening on the faster expanding part of my beam.

#### logsquared

##### Active member
Yeah its semantics. CDBeam is saying "multiply/ expand" the beam diameter (in that axis), therefore reducing the divergence. Its always reciprocal. Beam dia 2X = Divergence/2 and so on....

#### RedCowboy

##### Well-known member
Yeah its semantics. CDBeam is saying "multiply/ expand" the beam diameter (in that axis), therefore reducing the divergence. Its always reciprocal. Beam dia 2X = Divergence/2 and so on....
Kool, it gets the job done and makes for a lot more fun when you can focus desktop tight across the workshop, well with the c-lens pair and a beam expander, alas the zero mrad beam does not exist.

#### steve001

##### Well-known member
I've seen that myself with some laser diodes, I believe the NDG7475, and wondered what the hell was going on, but didn't think deeply enough about it. That is good it behaves that way, so when you correct the beam symmetry out of a laser diode with a cylinder pair, you are also reducing the divergence of the slow axis so the whole beam better matches as it travels into the deep distance. Thanks.
The 3X beam expander has 8mm in the clear at the input, however if you use the sanwu adaptor you need to drill out the little hole in the center, I have a dozen of them in use and have done this a lot, a 6mm beam is no problem.

That's what I used to think but ask paul, he has the master book and had explained that's not the case.

---EDIT---

paul would you like to chime in on this ?

Again I am reading what I always thought......we are talking about our edge emitting multi mode diodes are we not.....I would love to put this to bed but again I am going to call it the faster expanding axis and the slower expanding axis and the images I posted above are backwards of how it actually works, the narrow axis of the p/n junction has the higher divergence so why have I been told this is the slow axis ?

Here's a drawing.