correcting a NDG7475

[Alaskan]

I keep finding this subject mixed up on the net, some seemingly professional writes say the fast axis is the more diverging, I don't know what to think with this conflicting information.

More on slow and fast axis divergence here: https://photonlexicon.com/forums/sh...es-Fast-Axis-Slow-Axis-demystified-discussion

 

[RedCowboy]

paul has a highly rated laser book and he said........as I understood it, the term " slow axis " refers to the internal geometry/layout of the diode and not the rate of divergence so in our case with the multi mode N brand diodes the slow axis is the rapidly diverging axis rather the more aggressively diverging axis......I would like to say it correctly as well and believe this is the correct terminology in this case with these diodes.

 

[Alaskan]

Well, when referring to slow or fast axis, what you are telling me is the term fast axis is just referring to which axis has the quickest diverging beam out of the diode, prior to collimation, but that doesn't necessarily mean the collimated beam will have the highest divergence from the that side, and to the contrary, it has the lower divergence. I feel like swearing.... This is so easy to mix up. Yet, I believe we have seen diodes where the thin side of the stripe you see on a lens actually has the lower divergence, correct?

 

[RedCowboy]

The narrow part of the beam right out of your G2 is the aggressive axis and it overtakes the wider part of the beam straight out of your G2


You can even see it with a 3 element, just focus your laser to infinity and rotate your laser 90 degrees in the night sky, you will see that the narrow becomes wide and wide becomes narrow....well both are diverging and getting wider but you see the narrow overtake the wide, that's because what comes out of that tiny slot at the p/n junction in my pic above is a ( rapidly diverging ) ribbon and the thin part of that ribbon diverges a lot faster than the wide part, and it's that thin part that we use the c-lens pair to correct after using a G2 to get ahold of the whole thing which is all diverging rapidly, it's not at all like what comes out of a crystal, it's not at all a tem00 Gaussian beam, I wish it was.


---EDIT---
Yes the slow axis is the rapidly diverging axis in our case with these N brans diodes. IINM and I don't believe I am wrong but someone will disagree, they always do, but paul explained it and he has the laser bible book so I am going with that unless somehow I have it all screwed up, TERMINOLOGY WISE as far as shaping beams I have that, I do that, but this terminology debate has had me pulling hair as well.

 

[Alaskan]

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.

 

[steve001]

My concern is how wide the output of the beam is from an Optlasers 6X cylinder pair, if a 10X beam expander is used. They are difficult to find with a wide enough input aperture for that much expansion. For low divergence, I think it would be better to forget about the 6X cylinder pair to correct the beam symmetry and just use a large diameter PCX lens to collimate the output of a laser diode and live with the rectangle shaped output.

You could go the long route of using a cylinder pair to correct the output, then using a beam expander in front for a very nice laser pointer having that corrected beam shape, but the cost goes way up, unless you are fortunate to find something which will work cheap enough on ebay.

I've seen beam expanders that accommodate a larger beam diameter (>6 mm) input but they're not common it seems. The biggest hurdle isn't the lenses but the lens housing.

 

[steve001]

The narrow part of the beam right out of your G2 is the aggressive axis and it overtakes the wider part of the beam straight out of your G2


You can even see it with a 3 element, just focus your laser to infinity and rotate your laser 90 degrees in the night sky, you will see that the narrow becomes wide and wide becomes narrow....well both are diverging and getting wider but you see the narrow overtake the wide, that's because what comes out of that tiny slot at the p/n junction in my pic above is a ( rapidly diverging ) ribbon and the thin part of that ribbon diverges a lot faster than the wide part, and it's that thin part that we use the c-lens pair to correct after using a G2 to get ahold of the whole thing which is all diverging rapidly, it's not at all like what comes out of a crystal, it's not at all a tem00 Gaussian beam, I wish it was.


---EDIT---
Yes the slow axis is the rapidly diverging axis in our case with these N brans diodes. IINM and I don't believe I am wrong but someone will disagree, they always do, but paul explained it and he has the laser bible book so I am going with that unless somehow I have it all screwed up, TERMINOLOGY WISE as far as shaping beams I have that, I do that, but this terminology debate has had me pulling hair as well.

The fast axis = aggressive expansion (diverging) as you call it.
The slow axis = it takes it good ol' time expanding (diverging)

 

[RedCowboy]

I've seen beam expanders that accommodate a larger beam diameter (>6 mm) input but they're not common it seems. The biggest hurdle isn't the lenses but the lens housing.

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.

The fast axis = aggressive expansion (diverging) as you call it.
The slow axis = it takes it good ol' time expanding (diverging)

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

Read this>

broad area laser diodes

Broad area (or broad stripe) laser diodes are high-power laser diodes with a strongly asymmetric shape of the emitting region.

www.rp-photonics.com

---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 ?

 

[Alaskan]

Well, I saw that chart too, if the slow axis has a more aggressive collimated divergence it appeared to me some of the web sites have it backwards, not due to that chart, but due to how they write about it. I always thought the fast axis was the side with the higher divergence, once collimated. I don' see why the slow would have a greater diverging output, once collimated, but if that is what it is, that is what it is.

 

[RedCowboy]

I know how it works mechanically, it's peoples stupid dam understanding of the terminology that's all screwed up and I have heard it both ways, we are becoming a country of idiots because of this dumb shit with definitions that have 2 different explanations, I know things change but I say Qatar like Cut-Her well cutter like a box cutter, but the internet say it can be Cut Her or quote-tar as in getting a quote, I am again in doubt about the terminology but I know exactly how it works.



The beam from this tiny p/n junction expands faster horizontally as the image sits and slower vertically as the image sits on your screen, you want your diode in this orientation for the c-lens pair to sit on a shelf horizontally.

So why have I been told that the faster expanding axis the horizontal as this pic sits is the slow axis when I just read it's the fast axis ?

I would like an answer myself.

 

[Alaskan]

Maybe the confusion is in regard to how the beam spreads relative to the rectangle shape, in the distance, for most laser diode designs which has us all balled up.

 

[RedCowboy]

I don't know but I would like to say it correctly and not appear ignorant, however I have also been told and read it both ways, I'm afraid many people have learned it both ways, for now I am going to go back to saying the faster expanding axis and slower expanding axis which I hate.
I tend to believe what I just read in the link I posted, I am going to look for a definition at thorlabs as they do get it right, it's their business to know.

 

[logsquared]

paul has a highly rated laser book and he said........as I understood it, the term " slow axis " refers to the internal geometry/layout of the diode and not the rate of divergence so in our case with the multi mode N brand diodes the slow axis is the rapidly diverging axis rather the more aggressively diverging axis......I would like to say it correctly as well and believe this is the correct terminology in this case with these diodes.

View attachment 63607
View attachment 63608

EDIT this is in reference to the photos from above. That are not quoting for some reason

This is exactly backwards. Not sure, but Paul must have read the book wrong.

The wider part of the emitter has the " slower" smaller divergence. The narrower dimension has greater "faster" divergence.

I always thought the fast axis was the side with the higher divergence, once collimated.

You have this backwards.

This FA and SA are specified BEFORE the lens. Think about this.... Why would the nomenclature reference a device to lens that may or may not be used.

I don' see why the slow would have a greater diverging output, once collimated, but if that is what it is, that is what it is.

This is relatively simple to picture. Forget about the beam angles coming out of the diode for a minute. The emitter is a bar shape. Its a really wide bar (aspect wise) on the high power MM's. The far field spot is an image of the emitter.

So, if the image far away is a wide line, then the angle(divergence) from the lens to the width of line will be greater than the angle(divergence) to the narrower part of the line.

Hope this helps.

 

[RedCowboy]

EDIT this is in reference to the photos from above. That are not quoting for some reason

This is exactly backwards. Not sure, but Paul must have read the book wrong.

The wider part of the emitter has the " slower" smaller divergence. The narrower dimension has greater "faster" divergence.




You have this backwards.

This FA and SA are specified BEFORE the lens. Think about this.... Why would the nomenclature reference a device to lens that may or may not be used.



This is relatively simple to picture. Forget about the beam angles coming out of the diode for a minute. The emitter is a bar shape. Its a really wide bar (aspect wise) on the high power MM's. The far field spot is an image of the emitter.

So, if the image far away is a wide line, then the angle(divergence) from the lens to the width of line will be greater than the angle(divergence) to the narrower part of the line.

Hope this helps.

Yes I said that the smaller part of the emitter has the faster divergence, I said that paul told me that was the slow axis, that the slow axis was the faster diverging axis, others have said this too but I always thought otherwise and just read otherwise and posted the link.

We are not debating how it works just the terminology, it may well be how you look at the beam, it's like saying the left side of the car or the right side of the car, it's better to say passenger side or driver side.





The light emitting from the p/n junction seen through the facet expands faster vertically as this image is seen and slower horizontally as this image is seen.

The narrow axis should be the fast axis but I was told it's the slow axis by definition NOT by function, that's the issue, I think I was told wrong however it's not the 1st time on this definition.

 

[logsquared]

Yes I said that the smaller part of the emitter has the faster divergence, I said that paul told me that was the slow axis, that the slow axis was the faster diverging axis, others have said this too but I always thought otherwise and just read otherwise and posted the link.

We are not debating how it works just the terminology, it may well be how you look at the beam, it's like saying the left side of the car or the right side of the car, it's better to say passenger side or driver side.

Yeah Paul is wrong.

Im not sure how everyone got turned around. Probably like a lot of things on the interwebs one person posts it and it gets repeated.

The first picture on the link alaskan posted is right https://photonlexicon.com/forums/sh...es-Fast-Axis-Slow-Axis-demystified-discussion

 

[RedCowboy]

This is what I had always understood but got turned around on the definition, not the function, I prefer to say it as it behaves as it shows here.