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Reason for more powerful diodes having higher divergence.

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Hello, I apologize in advance for the dumb question. So, is the fact that the lens needs to be closer to the diode to capture all the light the reason that more powerful blue diodes with a higher raw divergence have a higher divergence after passing through the lens? According to my calculations, an NDB7875 at 8.0mm (the focal length of an Odicforce G8, as I can't find that spec on DTR's G8), with a divergence of 44°, should have a beam diameter of about 6.15mm in the fast axis, while the lens diameter is 6.5mm, which is really close to cropping the raw output. With a more divergent diode, like the NDB7A75, this should actually happen (correct me if I'm wrong), and you need to move the lens closer to the diode, thus increasing the divergence.
 



Wakrah

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For a given laser diode, if you collimate the beam closer to the diode, the output beam diameter will be smaller because the beam has not traveled far enough to spread out more (being the beam spreads after leaving the emitter), if collimated further away, wider. The wider the beam is when collimated to infinity focus, the less divergence. Longer focal length lenses are set to infinity focus further away from the diode than short focal length lenses.

In general, the thicker your starting laser beam, the more collimated it is, so if you manage to make a laser with beam starting at 1cm thickness, you'll have almost perfectly collimated laser beam.
 
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For a given laser diode, if you collimate the beam closer to the diode, the output beam diameter will be smaller because the beam has not traveled far enough to spread out more, if collimated further away, wider. The wider the beam is when collimated to infinity focus, the less divergence.

In general, the thicker your starting laser beam, the more collimated it is, so if you manage to make a (visible wavelength) laser with beam starting at 1cm thickness, you'll have almost perfectly collimated laser beam.
Does this mean what I said is right? You have to move the lens closer to the diode to prevent cropping resulting in a smaller initial beam and higher divergence? And if not, then how do people avoid significant cropping with diodes like the NUBM44 without a G2 short focal length lens?
 

Wakrah

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I'd ask this question in regard to a specific diode and lens combination, the focal length and diameter of the lens will determine if the beam is cropped or not when used with a specific diode.
 
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I'd ask this question in regard to a specific diode and lens combination, the focal length and diameter of the lens will determine if the beam is cropped or not when used with a specific diode.
Alright, what about a NUBM44 and a G8 lens?

EDIT: I did the calculations, assuming a focal length of 8.0mm (Odicforce G8), and a raw output divergence of 60°, a lens diameter of 6.5mm; 2tan(60°/2) = 1.155 (the correspondance between mRad and tan(x) decreases with high angles), and 1.155*0.008 = 0.00924m = 9.24mm, which is quite a bit more than the lens diameter, so there would be some significant cropping. There should be even more cropping with a 3-element lens, with an even higher focal length of 9.5mm and a smaller 5.0mm lens aperture (again from Odicforce). The power losses don't really reflect this, so either I did the math wrong or the lens would have to be closer to the diode, resulting in even higher divergence than a G2 due to the longer focal length, which also doesn't make sense. I'm completely stumped.

EDIT 2: It's possible that the Odicforce lenses are very different and have a significantly longer focal length. That might explain it.
 
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Wakrah

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Many members like to use the G2 lens for that diode, I'd be interested in knowing if the DTR G8 can be used with the NUBM44 without truncating some of the power. I have doubts, but if it can be used without loosing power due to clipping, that would be awesome. I believe RedCowboy has used that lens with this diode, maybe he will chime in.
 
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Many members like to use the G2 lens for that diode, I'd be interested in knowing if the DTR G8 can be used with the NUBM44 without truncating some of the power. I have doubts, but if it can that would be awesome. I believe RedCowboy has used that lens with this diode, maybe he will chime in.
I have read something about someone comparing the NUBM44 with G2, G3, G7, G8, and 3-element lenses. The measured power suggests there isn't much cropping. 5.2W with a 3-element vs 6.8 with a G2. That is right on par with the 70% rated efficiency for 3-element lenses, and 92% for G2 lenses.
 
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Thanks, but that doesn't say anything about whether the beam is cropped or not.
By the way, would a G2 lens have equal divergence to a an LFL lens if the initial diameter is equal? Because my gatling has some insane raw diode divergence for some reason, something like 120° or more. It hits like a milimeter of the heatsink, and almost a centimeter of the host itself. Even if I hold the lens flush to the heatsink, it still clips the output, so I'm considering a G2 to capture more light. Even with a G2 it might get cropped.
 
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Wakrah

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Focal length isn't related to diameter, it is the curve of the lens. You can have small diameter long FL lenses, or large diameter. Just depends upon the application which you would choose to use. With a 15 mm diameter lens I am using which has a FL of 30 mm, I can double the FL and still not completely fill up the diameter of the lens from one of my diodes, the output of other diodes would be clipped with that same lens.

The G2 has low loss for two reasons, the short FL allows all of the beam from the diode to be captured and being single element, there is less loss, compared to two or more elements glued together in one lens. Also a G2 has an aspheric lens shape which is a superior lens for collimating laser beams, compared to a standard PCX or double convex lens. For some diodes, using a G2 won't really do much for you compared to a G8, I am betting. Is the G8 aspheric too? The higher divergence of the G2 is a high price to me.
 
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Focal length isn't related to diameter, it is the curve of the lens. You can have small diameter long FL lenses, or large diameter. Just depends upon the application which you would choose to use. With a 15 mm diameter lens I am using which has a FL of 30 mm, I can double the FL and still not completely fill up the diameter of the lens from one of my diodes, the output of other diodes would be clipped with that same lens.

The G2 has low loss for two reasons, the short FL allows all of the beam from the diode to be captured and being single element, there is less loss, compared to two or more elements glued together in one lens. Also a G2 has an aspheric lens shape which is a superior lens for collimating laser beams, compared to a standard PCX or double convex lens. For some diodes, using a G2 won't really do much for you compared to a G8, I am betting. Is the G8 aspheric too? The higher divergence of the G2 is a high price to me.
I know it's not, I was talking about the raw beam diameter. Because it's insanely high, it will probably create a beam with the full width of the G2 lens, even with the short focal length. My question is, if the initial beam diameter is the same with a G2 and a G8 lens, would the divergence also be the same? For example, if the beam gets cropped with both lenses, having the same diameter as the lenses after passing through them.
 

Wakrah

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Under the assumption both lenses will indeed have some amount of cropping, the longer focal length lens will have more of the beam cropped than the short FL one, of the same diameter, so more power loss from cropping, if it is going to occur.

The problem is the lens holder tube may restrict the light from passing to the lens without being clipped as well as the diameter of the lens may also be too small to accept all of the light too, both together could cause a portion of the problem.

You could have such severe cropping by the ID of the lens holder barrel that 100% of the light won't even get to the lens anyway, or maybe the tube won't cut any of the light off, but as soon as the beam gets near the back side of the lens, it just then becomes too wide and some of the beam is cut off from entering all of the lens that way too. As a rule of thumb, I don't want the beam to take up more than 80% of the diameter of the collimation lens, some professional lens manufacturers don't even want that much.

I found with the NUBM44 diode, it is better to only have 50%, even less, of the diameter of the beam fill the lens, the beam charcteristics were far superior when I used only 25-50% of the lens diameter, at 25% I had the best result but I was using large PCX lenses, like several inches diameter. Until I started collimating the output of the NUBM44 with large diameter lenses, I always had wings on each side of the beam I didn't like.

Also, large diameter collimation lenses allowed me to focus the beam down to a burning spot 20 feet away, you cannot do that with a small 6 mm diameter lens. I am not the guy to talk about focusing beams to burn stuff, RedCowboy has tons of experience with the optics needed for that.
 
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Under the assumption both lenses will indeed have some amount of cropping, the longer focal length lens will have more of the beam cropped than the short FL one, of the same diameter, so more power loss from cropping, if it is going to occur.
I know, that's why I'm even considering the G2 lens. I'm curious about the divergence in that case.
 

Wakrah

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With the NUBM44 diode, the divergence is strait from hell, God awful, don't even consider it unless you don't care about having a huge rectangle shaped beam just 50 feet away. I don't even like the divergence using that lens with a NDG7475 green laser diode which has far less divergence, about 1/4 of the NUBM44
 
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With the NUBM44 diode, the divergence is strait from hell, God awful, don't even consider it unless you don't care about having a huge rectangle shaped beam just 50 feet away. I don't even like the divergence using that lens with a NDG7475 green laser diode which has far less divergence, about 1/4 of the NUBM44
I'm not considering it. I want it for my Gatling, which for some reason has like 120° of raw output divergence, getting cropped even with the lens touching the heatsink, therefore even a G2 will produce a wide initial beam. The divergence after the lens is just fine though, around 2mRad.
 

Wakrah

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I guess the diode in your Gatling is set back a bit, too far back to collimate to infinity with that lens, the G2 has less than a 3 mm focal length to reach infinity focus, 2 point something, thus the "2". I should know, I'm loosing it, forgetting. I can now see why you want a shorter FL lens.
 




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