Low power single mode vs high power multi mode laser diode divergence?

[Alaskan]

OK,

I've googled and searched for hours for an answer and can't quite find what I'm looking for, so posting the question here. Trying to choose which is better when considering a relatively low power single mode blue laser diode such as the PL450B, or the Nichia NDB7875 multi-mode high power diode, which of the two will produce the highest intensity spot at a great distance, when considering divergence?

My understanding is when using a single mode diode it can actually out-perform a multi-mode diode at a great distance due to less beam-spread. However, there must be a trade-off between power too, anyone have any idea how to figure this? Will a 3 watt NDB7875 still deliver more power for a given area at a long distance than a single mode laser which has 1/10 the power output? I don't know the divergence which can be had from a single-mode PL450B diode or what the divergence can be for the NDB7875 multi-mode to be able to get a rough idea, I suppose that is all I really need to know to figure this out.

Why?

I'm still working on the idea of combining the output of several individual laser diodes, each with their own collimation to produce a brighter light when viewed at a distance. My reasoning: Since there will be several laser diodes all arranged together shooting beams of light out in parallel, each beam will slowly diverge which will of course cause all of them to merge into one another but when they merge or spread out into one another, their powers will add together to produce a brighter light, as viewed from many miles away. This is my assumption and why I want to use multiple laser beams instead of one, well that and I can't just go buy a 25 watt 445nm laser diode.

I'm also constrained by the cost of the devices used, so that is also a factor. Maybe I can get more power per square inch etc. using single mode but how many of them are needed to produce the same amount of power a multi-mode laser diode can produce? Less of them, more? I'm guessing I'd need more single-mode 200mw PL450B diodes to be able to compete with what a high power 3 watt NDB7875 can do, but this is wholly a guess without knowing more about the possible divergences between the two (without using a beam expander).

Anyone here with experience with either of these diodes who can comment on the divergence they were able to get?

 

[Down with Umbrella]

The PL450B might have tight divergence but you might not get the visibility you are looking for. For power per square inch the BDR 405 16X is single mode and pushing almost a Watt however, that wavelength is generally pretty dim Despite its power.

If you're going for brightness especially with viewed from further away I would go with the 9mm NDB7875. Beam collmantion is done in a Variety of ways (PBS cube, knife edge, etc.)
How are you planning to "merge" these beams in parallel?

Compromise with the PLTB450B? Could suite your purpose.

 

[Alaskan]

No special optics, just a bunch of pointers bundled together on a heat sink. When I said merge, I just meant as the closely grouped parallel laser beams each become wider at a distance, solely due to divergence, they will begin to overlap one another as they travel forward and from this, their individual power will add together to make the distant spot much brighter. In effect, becoming a fat beam which continues to diverge, as normal, as it goes forward.

At some point the individual beams will become indistinguishable and appear to be one big fat beam from each of them getting wider. I want to do this to overcome the loss caused by divergence, as a kind of cheat. Of course, no freebie here, just more parts and power. I posted a question on this once before, but wasn't deciding between different diodes, single vs multi-mode. I was considering the PLTB450B too simply because the price is so good for them but aren't those a multimode version of the PL450B?

 

[Down with Umbrella]

Alright, I see the idea. I'm not sure how it will turn out :thinking: Without corrective optics the beams will cross at a certain point then separate if not lined up right... But I would imagine if they were mounted close together parallel the divergence would over lap and appear as a fat beam.

Yes the PLTB450b is multimode but has better divergence than an M140(which is better than the 9mm445 ndb7875 ) it output is roughly 1.9W so it's a trade off.

 

[Alaskan]

Lining the beams up to be perfectly parallel using DTR's copper laser diode mounts with a screw in G2 lens turned out to be a problem for the last multiple laser beam unit I built because the lens holder threads and threaded copper diode mount aren't super precision (copper is soft) causing the lens assembly to produce slightly different angles in the barrel, from laser to laser. A slightly different angle turns into a big difference a few hundred feet+ out. To solve this I was looking at a full plate of factory mounted NDB7875 laser diodes with integral lenses out of a projector but their divergence isn't as tight as the PLTB450b diodes, maybe I will just go ahead and buy those to make the project simple, although not as optimum as I'd like.

Then again, if I buy the loose individual PLTB450B's instead of the factory made plate of 12 laser diodes with lenses, I can have the freedom to incorporate a beam expander lens into the assembly to reduce the divergence even more... but more cost and complexity for machining the thing. So, without more information I can't make a decision yet, need to know what I can expect for divergence from the different diodes.

Do you have any rough idea what the divergence is for the PLTB450B and the NDB7875 diodes using a G1 or G2 lens?

Has anyone built a beam expander which uses the G1 or G2 lens? I have purchased a bunch of small concave lenses I was hoping could be incorporated into a lens assembly to reduce divergence but don't have examples of what others have done for something like that.

 

[Cyparagon]

which of the two will produce the highest intensity spot at a great distance, when considering divergence?

That depends on the distance. Up to a certain point, the higher power blue will have higher power density.

Remember your high school algebra? Time to dust it off. Set up two equations (one for each diode) and two variables (power density and distance), then find out at what distance the power density is equal. Assumptions for single mode: round and top-hat beam profile, 4mm 150mW initial beam diameter, and 0.5mRad. Assumptions for multi-mode: rectangular and top-hat beam profile, 4x1mm 1.8W initial beam diameter, and 1x3mRad.

Diameter at a distance for single mode:
Diameter = 4mm + (distance in meters) * 0.5mRad
Power_density = 150mW/Area
Power_density = 150mW/(pi * (Diameter/2)^2)
Power_density = 150mW/(pi * ((4mm + (distance in meters) * 0.5mRad)/2)^2)

For multi-mode:
Diameter_fast_axis = 1mm + (distance in meters) * 3mRad
Diameter_slow_axis = 4mm + (distance in meters) * 1mRad
Power_density = 1800mW/Area
Power_density = 1800mW/(Diameter_fast_axis * Diameter_slow_axis)
Power_density = 1800mW/((1mm + (distance in meters) * 3mRad)*(4mm + (distance in meters) * 1mRad))

If we want to know the distance at which they are equal, we just set them equal:
150mW/(pi * ((4mm + (distance in meters) * 0.5mRad)/2)^2) = 1800mW/((1mm + (distance in meters) * 3mRad)*(4mm + (distance in meters) * 1mRad))

Remove units and let distance in meters = x to simplify:
150/(pi * ((4 + 0.5x)/2)^2) = 1800/((1 + 3x)*(4 + 1x))

It gets a little bit hairy after this, so just ask wolfram alpha like so.

It even gives you a nice graph of each power density with distance. In this case, 44 meters is the break-even point. Feel free to change the values/assumptions and see where the break-even point lies. Ignore any negative numbers. If you ever get to a point where there are only negative/imaginary answers, that means that the multi-mode will always dominate.

An example would be if we corrected the fast-axis on the multi-mode to 1mRad

 

[Down with Umbrella]

Thanks Cyparagon for the data & and formulas. Excellent info.

 

[Alaskan]

Thanks, I've been reading this and looking at the links for a day now, wanted to wait to respond. Let me see if I understand this right, the 44 meter break even point means that at that distance, one diode is as bright as the other or both have the same intensity? I have to admit, I'm not catching on well yet, which diodes output is brighter past 44 meters? The 150mw single mode or the 1800mw multimode? Intuitively, my guess is the high power multimode will beat the pants off of the 150mw single mode for a long distance but the math is the reality, of course. Maybe my intuition is completely bass akward on the reality.

I am so thankful you put so much time into your response to spell things out for me, I will continue to ponder your answers, but I'm being a blockhead on this subject, it seems.

 

[Cyparagon]

which diodes output is brighter past 44 meters?

The single mode will have higher power density beyond 44m and the multimode will have higher power density before 44m (in theory, and given the assumptions I've made).

 

[DTR]

One thing on the blocks. If you have ever seen one running out of the projector if you point it at a far away target you will notice the beams are going everywhere and you would think they would line up nice and even but they don't. They really only need to hit something that is several inches away so I guess precision is not key there.

On the 12mm modules with lenses most that use them in projectors use teflon to remove any play out of the lens where in a portable laser just a spring is fine as you don't notice very slight angles of the beam that would make a huge difference when combining beams.

Also thought you had told me you wanted to make a dot miles away or is it only 40-50 meters?

 

[Alaskan]

Thanks again,

Yes, I want to point at something miles away and have the maximum beam power delivered to it, the example given was to show where the two diodes; a single mode and a much more powerful multi-mode diode have equal power, amazing to me that a 150 mw laser produces the same power density of a 1800mw diode at only 44 meters away, thanks again your awesomeness Cyparagon.

DTR, thanks for the info on the diode blocks straight out of a projector, I had thought they would be all lined up perfectly, so appreciate the word on that. On the copper headed diode mounts I remember you had told me to use Teflon tape to make the lens holders more secure but I never went out again with my 16 diode 455nm machine after that to do so. I sure can't see any error in their pointing looking at them, but small errors add up when shooting the beams thousands of feet which cannot be seen closer in. The small error would never be noticed in a single pointer as you just couldn't tell without a reference.

 

[upaa27]

http://www.buildlog.net/cnc_laser/laser_calcs.htm Really nice laser optics calculator

 

[Alaskan]

Thanks for the link, knowing how MUCH cutting the divergence in half or so can make for the distance a laser beam can go (for a brighter spot) opens up a lot more diode candidates for the project. Now I have to go searching the lower powered diodes available to see how low a divergence I might be able to get. I wonder how reasonable it is to expect .5 mrad out of a single mode laser?

 

[Rifter]

Thanks for the link, knowing how MUCH cutting the divergence in half or so can make for the distance a laser beam can go (for a brighter spot) opens up a lot more diode candidates for the project. Now I have to go searching the lower powered diodes available to see how low a divergence I might be able to get. I wonder how reasonable it is to expect .5 mrad out of a single mode laser?

You might get that from a lpc-826. Mine has a noticeably better divergence than my pl450b which is supposed to be 1mrad or so.

 

[Alaskan]

1mrad is great, but I was dreaming of something closer to half, am I dreaming because I'm not going to be able to find a visible light laser with that tight of a beam without expansion?

 

[Rifter]

1mrad is great, but I was dreaming of something closer to half, am I dreaming because I'm not going to be able to find a visible light laser with that tight of a beam without expansion?

See if a lpc-826 is tight enough, I've been told it's the lowest divergence diode that's readily available. My pl520 also is good. Can see dot on clouds with both my lpc-826 and pl520 but not my pl450b. Since pl450b is supposed to be just above 1mrad I would bet the pl520 and lpc are both sub mrad.