Summary of factors affecting our ability to use diodes for a nice beam?

With all of the experimentation into different diodes we've had around here recently, I'd love to have a more thorough understanding of the factors that affect our ability to use diodes for an attractive beam.

Q1) Obviously the divergence of the two axes is important. But is it the absolute value difference between the axes that matters? Is it the relative difference? Does the magnitude matter?

- So for example, would divergence specs of 5,10 degrees for the two axes be on par with specs of 15,20 - since they have the same numerical difference?
- Or would 5,10 be on par with 10,20 - since they have the same ratio relative to each other?
- And presuming a wonderful diode with equal divergence on both axes, would 5,5 be any better/worse than 10,10? what about 40,40 ?

Q2) I've heard the terms:
- Single Mode
- Multi Mode
- Multi Mode Single Emitter

It's the latter that has me confused. I was under the impression that (and this is far from technically precise) "multi-mode" indicated that there were a bunch of emitters really close to each other, as opposed to just one. But this understanding can't be correct if it's possible to have a "multi-mode single emitter"

Presumably single mode is preferable. What is it about multi-mode 445s that allowed them to be useful for our purposes? Or more to the point - what qualities would indicate in other multi-mode diode specs that they could be similarly useful for our purposes?

Q3) Emitter size. Presumably smaller is better for a tight beam, but larger is often able to handle more power? Is there such thing as a "good" size? 50 um ? 150 um ? Or is that entirely dependent on other characteristics of the diode and wavelength?

Just some of my lingering questions.

OMG I wish I knew what you just said, now I realy feel like a hillbilly and I know I can't spell woth beans :crackup:

rhd said:

With all of the experimentation into different diodes we've had around here recently, I'd love to have a more thorough understanding of the factors that affect our ability to use diodes for an attractive beam.

Q1) Obviously the divergence of the two axes is important. But is it the absolute value difference between the axes that matters? Is it the relative difference? Does the magnitude matter?

- So for example, would divergence specs of 5,10 degrees for the two axes be on par with specs of 15,20 - since they have the same numerical difference?
- Or would 5,10 be on par with 10,20 - since they have the same ratio relative to each other?
- And presuming a wonderful diode with equal divergence on both axes, would 5,5 be any better/worse than 10,10? what about 40,40 ?

Click to expand...

Generally speaking, it's the ratio that matters. If the beam is a circle, it matters less how fast it is diverging, optics can generally handle it. We run into trouble with diodes because the ratio between the divergences is high. You have to correct one axis to collimate the light well.

rhd said:

Q2) I've heard the terms:
- Single Mode
- Multi Mode
- Multi Mode Single Emitter

It's the latter that has me confused. I was under the impression that (and this is far from technically precise) "multi-mode" indicated that there were a bunch of emitters really close to each other, as opposed to just one. But this understanding can't be correct if it's possible to have a "multi-mode single emitter"

Presumably single mode is preferable. What is it about multi-mode 445s that allowed them to be useful for our purposes? Or more to the point - what qualities would indicate in other multi-mode diode specs that they could be similarly useful for our purposes?

Click to expand...

This ties into your next question about emitter size as well, because for the diodes we have here, the emitter size is what largely determines the mode behavior.

All the common diodes here are single emitter. Optical storage diodes (DVD, BluRay, etc) are all single mode because the application requires it. The 445nm diodes are multimode, but are still a single emitter. There are multiple modes in which the light is confined, but still within one waveguide. Laser bars do exist where multiple emitters are combined next to each other on one chip, but I've never seen one used on this website that I can recall.

"Multimode" is all relative as well. Think of it as, the wider an emitter, the more modes "fit". Single mode diodes are less than 2um wide, because any wider and they would be multimode. The 445nm diodes here are only 15um wide, so multiple modes fit, and for some other reason(s), the modes are very well-behaved and predictable. But 15um is still fairly narrow, so it's still relatively few modes. Wider ridges, going all the was to broad area lasers, have more and more lateral modes. The 445nm diodes are very well-behaved multimode lasers, because they are still quite narrow, and some other reason(s) I don't fully understand.

This answers your next question: width depends on purpose. Optical storage requires a narrow ridge because it requires single-mode for tiny spot size focus. More power requires a wider ridge, but you get multimode behavior, IF that is ok for your purpose. The 445nm diodes only have to fill a DLP chip with light, so multimode is fine for that, and they are well-behaved enough to get consistent performance, even though they are mutlimode.

rhd said:

Q3) Emitter size. Presumably smaller is better for a tight beam, but larger is often able to handle more power? Is there such thing as a "good" size? 50 um ? 150 um ? Or is that entirely dependent on other characteristics of the diode and wavelength?

Just some of my lingering questions.

Click to expand...

A small emitter is preferable to get single-mode operation, yes, if that is what you desire. If you need more power, you need a wider emitter. 50 and 150um are both LARGE sizes, basically broad-area, and would not be very well behaved at all. It's all about engineering the proper compromise for your purposes. In general, the narrower the emitter, the better the beam is likely to be.



Remember for the future also: divergence is inversely-proportional to aperture. You get asymmetric laser diode emission because the transverse mode is small and the lateral mode is wide, making high divergence in the transverse direction and lower divergence in the lateral direction. Then, the wider your emitter (in the lateral direction), you get multimode behavior in that lateral direction, which further complicates things.

rhd said:

With all of the experimentation into different diodes we've had around here recently, I'd love to have a more thorough understanding of the factors that affect our ability to use diodes for an attractive beam.

Q1) Obviously the divergence of the two axes is important. But is it the absolute value difference between the axes that matters? Is it the relative difference? Does the magnitude matter?

- So for example, would divergence specs of 5,10 degrees for the two axes be on par with specs of 15,20 - since they have the same numerical difference?
- Or would 5,10 be on par with 10,20 - since they have the same ratio relative to each other?
- And presuming a wonderful diode with equal divergence on both axes, would 5,5 be any better/worse than 10,10? what about 40,40 ?

Q2) I've heard the terms:
- Single Mode
- Multi Mode
- Multi Mode Single Emitter

It's the latter that has me confused. I was under the impression that (and this is far from technically precise) "multi-mode" indicated that there were a bunch of emitters really close to each other, as opposed to just one. But this understanding can't be correct if it's possible to have a "multi-mode single emitter"

Presumably single mode is preferable. What is it about multi-mode 445s that allowed them to be useful for our purposes? Or more to the point - what qualities would indicate in other multi-mode diode specs that they could be similarly useful for our purposes?

Click to expand...

Having played with many lenses, on many types of Laser diodes,

the optics can get quite confusing.

A divergence of 15-20° (3 to 4 aspect ratio) would usually be preferable to 5-10° (1 to 2 aspect ratio).

The closer the divergence angles, the better the beam after simple optics.

A 5-5° laser diode would be a great beam ! !

Modes refer to specific frequencies of operation.

When a single diode goes into multi-mode operation,

it produces more than one focal point.

The bar diodes combine several individual laser diodes into one wide emission pattern.

This makes the beam difficult to collimate into a desired single dot.

LarryDFW

That was a phenomenal explanation - and I think I followed most of it too (until the very last line).

I take it that in terms of the diodes we encounter here, Multi Mode and Multi Mode Single Emitter are probably just different ways of referring to the same thing? (since we never really encounter multi-mode multi-emitter diodes)

So to apply what I've learned to a diode that I'm been pondering:
A) 2,7 beam specs (after FAC added)
B) 60 um emitter size
C) Multi mode single emitter


A) My analysis would be that the beam specs are workable. The ratio between them is about the same as what I've been told the 445s are (12,40). However, the absolute value is much much lower. If I understand correctly though, this shouldn't be any more/less of a problem roughly speaking.

B) The emitter size here is quite large. That may be a deal-killer if the purpose here is vanity (IE, creating a laser pointer with a nice beam *** and with no technical function other than to stimulate my creativity and give me a weekend project )

EDIT: Having said that, I'm also sort of inclined to think that the lower absolute value of the divergence figures may compensate slightly for the large emitter size. My logic being that the beam may start out larger, but it won't grow as much by the time it hits the lens. Whereas a 445 might start from a smaller emitter, but grow more rapidly before being tamed by a lens. Apologies for the non-scientific terminology. Does that notion hold up at ALL ?

C) Multi-mode single emitter isn't really determinative of anything. It's the combination of other specs ^ that I should worry about.

Does this sound reasonable?

60um diode bare output would likely give you a hard-to-tame beam because of asymmetric emission, but as you say, it has FAC already. The whole point of fast-axis correction is to make the divergence more symmetrical and the emission easier to collimate, and it seems to work with this diode, given the divergence numbers.

So the asymmetry is at least partially fixed by the FAC here, somewhat negating the worries about asymmetric divergence.

So when you say asymmetric divergence - does that mean more than just that the axes diverge at different rates? Because it seems to me like the axes here still diverge at different rates (2,7). Are you just saying it is somewhat negative because they are closer to each other?

The pre-FAC specs in this case would have been 50,7

Part of my motivation in wanting to learn more about divergence is to enable me to evaluate whether the purchase of a 622nm, 9mm, w/ FAC, 7x2 degree, 60um, 150mW diode would workable in a 9mm housing without non-standard optics. The color would be phenomenal:
Wavelength to RGB / HEX Calculator - 622nm

It's a fairly pricey investment, so I was hoping for input. Thank you to everyone who pitched in on this thread

I'm still not entirely settled on whether it would make sense to move forward, but I'm a bit closer to that state now.