Multiple laser diodes prior to collimation?

[Alaskan]

Then it is back to using one single laser diode, a tighter divergence is number 1, everything else second.

I have another way of doing it, I ordered a large number of S1 lenses, I can combine a bunch of collimated laser diodes all in parallel to make a fat beam composed of many individual two watt blue lasers, at a distance the divergence will cause them to all widen out into each other anyway, adding to the power of the spot, as it widens. Of course, still a net loss at a distance due to divergence, but not as much. I'd rather have a single high power blue laser but I can't afford one. I don't want to deal with a bunch of knife edges to do it either, I want this unit to be hand held, heavy as it will be, but only needed for a few minutes of operation, max 3 second bursts with a couple of seconds between bursts for no more than 15 seconds at a time with 1 or more minutes of rest between cycles.

I would consider a knife edge arrangement if it can all fit in a 3 to 4 inch diameter tube and produce tens of watts of power output but I don't see how to do that yet, but I'm going to start looking into that as a possibility. Just don't want a big heavy square box.

I know this is going to rouse some "what the hell for" questions, you will hurt someone, go to jail etc if you hit an airplane with it... I won't do that.

 

[VisibleGreen]

Would be awesome if we could do this

 

[KrowBar]

There is a fundamental flaw with your original idea. In order for a collimating lens to produce the desired effect (a beam of parallel rays) the source of diverging rays must be placed at the focus of the lens. Since your lens will have a single focal point, you can't have 3 distinct ray sources simultaneously collimated by that lens. There was a previous discussion where someone wanted to do this to create a lightsaber http://laserpointerforums.com/f50/multiple-16x-red-diode-light-saber-51161.html. You can see that this guy refused to accept that you can't combine beams this way. You can always focus light from several sources on a common location and get a combined spot like you see in your pictures with the overhead lights, but that is a spot at a particular distance - you won't get a parallel beam by putting any type of lens at that spot either. Your idea of just having several beams with some noticeable separation is sound, but like you said, the divergence will not be any better than a single narrow beam.

Maybe if you let us know your application, we can help you out with other ideas. So far it sounds like you want a high power spot at a significant distance to be produced by a single handheld device ( tell me if I am reading you correctly) that will be on fr only about 3s at a time. What power density do you need at the target? and what size of spot? Are there other requirements besides the specs at the target (like the beam must be a certain color or at least visible and the shape of the beam is important etc.)?

 

[Alaskan]

Thank you for your input, appreciate the feedback on what I'm attempting and the problems. I will PM you more detail.

To answer the questions I am open about:

So far it sounds like you want a high power spot at a significant distance to be produced by a single handheld device ( tell me if I am reading you correctly) that will be on fr only about 3s at a time.

Yes, that's about it.

What power density do you need at the target? and what size of spot?

I have not worked out a power density requirement, the size of the dot is important in that the smaller it is, the brighter it will be at a distance. At close distance of under a mile the power output of several watts is not needed and in fact is too powerful. I will build in IR diodes for such use.

Are there other requirements besides the specs at the target (like the beam must be a certain color or at least visible and the shape of the beam is important etc.)?

The laser power will not require a reflected return, just visible at a great distance by either CCD or normal vision. Please keep the other details I passed on in PM between us. I don't like mentioning in public some details of the project are private, but I have reason to do so. If other members are curious just PM me and ask, I just don't want it posted.

Thanks!

 

[KrowBar]

When talking about large distances, sometimes having a huge divergent beam with high power allows you to be out of alignment and still detectable, but a high quality low power beam can still have superior power density - just not as tolerant of mis-alignment.

 

[Alaskan]

Good point, I should have both. Some diodes dedicated to as low a divergence as I can get, others with a wider divergence.

 

[BShanahan14rulz]

fiber launching, your divergence probably depend the most on the size of the fiber relative to the size of the collimating lens. Generally speaking, and assuming well-made lenses, the larger the lens, the lower the divergence.

But benefit is that if your fiber can handle the power, you can feed one end like in your spotlights picture. Of course, easier said than done. Haven't seen anyone bother with this yet on the hobbyist level.

See if retroreflectors help in your application, too.

 

[Cyparagon]

Not gonna work. Your "emitter size" effectively increases to the distance across all the emitters. Divergence in any light beam is proportional to the emitter size - it's why you'll never see a fluorescent lamp in a searchlight.

It's shite even if the emitters are directly next to each other (like in a 40W diode bar for example). If you're talking about putting a couple diodes next to each other, you'll just get two beams.

 

[Alaskan]

Thanks again, sounds like the best way to put more light out at a distance, for my budget, is to parallel several individual lasers together with as tight a divergence as I can get for each one, that way, their individual divergences spread out into one another to add to the total power per square area of illumination.

For example; I might see 25 individual 2 watt beams going out, but at a very long distance they diverge out to make one more powerful fat beam which to an observer appears to be 25 times brighter, compared to one. You are still loosing power at the same rate due to divergence, just that you are starting out with more. To me, this is a kind of a cheat, but not for free.

I wonder if I could somehow end up with the same product anyway, if using one lens for several laser diodes closely grouped behind it and then paralleling a bunch of those so they add together the same way. Perhaps bundling several low mrad lasers together is the most cost effective solution to produce maximum photons on a target at a very long distance, what do you all think? (as opposed to a higher power single laser which can produce the same amount of light at a distance).

Even more power at a distance could be achieved if each of the 25 low mrad lasers in parallel had beam expanders on all of them, i.e. 10x beam expanders, then the divergence would be cut down to 1/10th for each laser delivering about ten times more light at a distance compared to a laser without it. That could be one awesome spot beam. Would that produce 250 times more light on a target than a single laser beam of the same power without a beam expander? Is that how it would add up?

 

[KrowBar]

You have the right idea there about the total power per area on a distant target. However,

I wonder if I could somehow end up with the same product anyway, if using one lens for several laser diodes closely grouped behind it and then paralleling a bunch of those so they add together the same way. Perhaps bundling several low mrad lasers together is the most cost effective solution to produce maximum photons on a target at a very long distance, what do you all think? (as opposed to a higher power single laser which can produce the same amount of light at a distance)

will not give the same result as a bunch of individually collimated beams - As mentioned earlier that gives a very large effective emitter. The beam diameter X divergence is more or less constant for a given beam. If you were to pull the lens off a typical 1mm, 1mrad laser you would see something like 25 deg. divergence from the emitter. Since that divergence is like 500 times the 1 mrad of the 1mm beam, that means it is coming from an origin of something like 2 microns (that would be the emitter size). Now put two diodes as close together as possible - lets say somehow you have high power diodes in 3.8 mm cans - Your effective emitter size is now 3.8 mm instead of 2 microns with the same initial 25 deg. divergence. That means to get your 1mrad divergence, you have to get to 1.9m diameter. You are now looking for a collimating lens with a F.L. of 4 meters and a diameter of 2 meters. So...

it's why you'll never see a fluorescent lamp in a searchlight

Stick with multiple overlapped beams if you need more power per area at a distance - you won't start things on fire from 100 yards with that method, but your buddy on the mountaintop 50 miles away will have an easier time seeing you try to shine your laser in his eyes if you have 25 of them pointed at him at once.

Another thought - The best optical arrangement for someone to see your laser from a distance would involve a beam expander on your end coupled with a beam de-expander (read telescope) on their end. Just as you can see fainter stars by using a telescope, you can see less powerful lasers pointed at you as well.


[edit] This is not to be taken as a suggestion for anyone to look into lasers with telescopes!

 

[Alaskan]

Apologies for the late reply, I've purchased a large number of single mode laser diodes inside 12mm X 30mm modules and going to make a build with all of them in parallel. Wish I could find a way to modify these modules so they could have a beam expander built inside, I found some small concave-concave lenses on ebay that I'm research how to add them to the assembly, don't know if such is possible yet though, with the distances needed between the expander and S1 lenses I have.