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RedCowboy

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Focal length doesn't explain why the beam's not staying square.
 



Coonie

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If you read the thread he actually says the beam is square but the dot is rounded. You need a long focal length lens or the divergence sucks. You guys are confused about FAC lenses so it's why I posted that thread as it helps explain things and lets you know what to expect.
 

logsquared

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Focal length doesn't explain why the beam's not staying square.
Read my post #97. I explain it there. The rough example at the bottom is for a m140. The diode you have probably has 2 to 3X the emitter width. You can't have a square beam at the lens and at the far field spot with your diode. The FAC can make it better (more square) by what ever ratio the FAC changes the raw divergence. From laser tree spec it looks to be around 4-5 times.

If you put a 3E lens on the version of you diode without a FAC and the spot on the wall at 20 feet is say a 2mm X 50mm bar. Then the FAC'd version with the 3E *should* produce roughly a 10mm X 50mm bar.

The diodes they sold as "linear" had much greater reduction in the FA raw divergence. So with the same diode with the "linear" FAC and a 3E lens the spot at 20 feet should be more square like 40mm X 50mm or so. This is unless the "linear" FAC is fully collimated. If fully collimated the 3E will not be able to focus the FA and SA at the same time and a cylinder will be used to collimate the SA.
 

CDBEAM777

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Thank you all for your contributions !!! It will take me some time to understand them all !!!
SO....I read...and struggle to understand....Sometimes...briefly...I think I do understand....then
, I am grounded and humbled. again...So it goes.

SO...I tried a 12.5mm dia / 75mm EFL Convex lens from THOR LAB to tame the Blue FAC
NO Luck !!! Sounds like no one else has
achieved a solution....so...I do not feel so bad !!! I will keep experimenting !! CDBeam
 

logsquared

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SO...I tried a 12.5mm dia / 75mm EFL Convex lens from THOR LAB to tame the Blue FAC
NO Luck !!! Sounds like no one else has
achieved a solution....so...I do not feel so bad !!! I will keep experimenting !! CDBeam
My guess would be with a 75mm FL the SA should be around .5mRad and the FA around .1mRad.

God only knows what they glued on there and called a FAC.

What did the beam look like with the 75mm FL? Im very curios. Also, did the beam fit on the 12.5mm dia?
 

CDBEAM777

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LS...OK....I tried various 75mmEFL positions...10mm from the LD output....to 150mm distance...The LD had a G8 Colli lens attached when the test was made.

Up and down the C/L....ALL CRAP!!! Least geometry achieved was about 100mm Square @ 14M Distance To Target.

The beam output from the G8 fit on the lens....for the full range tested. After seeing this was a Dead End....Not anymore time spent on
documentations....as it was NOT going to be viable....Sigh.....

CDBeam
 

logsquared

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LS...OK....I tried various 75mmEFL positions...10mm from the LD output....to 150mm distance...The LD had a G8 Colli lens attached when the test was made.

Up and down the C/L....ALL CRAP!!! Least geometry achieved was about 100mm Square @ 14M Distance To Target.

The beam output from the G8 fit on the lens....for the full range tested. After seeing this was a Dead End....Not anymore time spent on
documentations....as it was NOT going to be viable....Sigh.....

CDBeam
Ditch the G8. use just the 75mm FL.
 

Philipnzw

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Okay, let me preface this by saying that i'm absolutely trash at physics and i have very basic understanding of optics.
So, according to what i've learnt from reading posts on LPF, the general rule is "the longer the FL of your colli lens, the lesser your divergence, at the lost of getting a fatter beam."

So i guess these FAC diodes will only be good for thick beam laser pointers? We could use lens such as this.

20mm lens 47mm FL

16mm lens 40mm FL

Lens holder for 16mm lens

Just my 2 cents, sharing what i know.
I'll likely get the 16mm lens just to mess around with it, with the hopes that in the future i'll make a NUGM03 pointer with a thick beam.
Gotta hit those cloud bases.
 
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Singlemode Laser

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Focal length doesn't explain why the beam's not staying square.
Like allready stated before this is not possible with multi mode diodes. They have a large M^2 for the slow axis:


The M^2 is in order of 5-10. So for the same waist size the divergence will be 5-10 times higher than the fast axis. A square beam just can't stay square over distance. Only an elliptical one can.

With single mode diodes a round beam can be achieved with cyl. lenses f.e.


Singlemode
 

paul1598419

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Yeah, this is what I believed would be the case with these blue diodes. I can't understand exactly what the producers of these "FAC" corrected diodes had in mind by doing this, but I was dubious that it would turn out to be something that would give us a better beam quality. Unfortunately, it is what it is.
 

Singlemode Laser

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Yeah, this is what I believed would be the case with these blue diodes. I can't understand exactly what the producers of these "FAC" corrected diodes had in mind by doing this, but I was dubious that it would turn out to be something that would give us a better beam quality. Unfortunately, it is what it is.

I can imagine something like CNC applications, where you are interested in beam profile at one point only (in the focus) then these FAC diodes help you if you don't want any additional optics, otherwise I don't know.

Singlemode
 

logsquared

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Don't give up yet! The first testers have only had them for a few days.

I doubt laser tree had pointers in mind when then FAC'd these diodes. That doesn't mean they can't be used though.

Also, about using aspheric lenses to collimate these..... Be careful buying any old aspheric lens especially if its expensive. The lenses can have many different profiles and may work worse than a regular spherical lens. I would wait till some of the guys who have these try a few lenses before blowing $ on something that may not work.
 
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logsquared

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You can't beat the laws of physics. There is no such a thing as the multi mode diodes we know with a round beam staying round over long distances.

Singlemode
I know that go back and read my posts. I have been trying for 10 pages to explain how these work.

But it should offer much improvement with simple optics. Much more compact and easy to build compared to cylinder pair.
 




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