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What's the smallest spot diameter at 2000m distance that $5000 will buy ?

Zoomby

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What's the smallest spot diameter at 2000m distance that $5000 will buy ?

For a Gaussian beam profile let's assume the radius of the spot to be where its intensity falls off to 14% (1/e^2) of its on-axis intensity.

Power: ~1mW
Wavelength: Any (including non-visible)
Maximum beam diameter at the output optics: 200mm
Beam profile: Can be Gaussian but a more tophat one with sharper edges is preferable.
Physical size of the entire device: Any (does not need to be portable)
Power Supply requirements: Any.


Note, that I am not asking for the lowest divergence beam at that price but for the smallest spot diameter at 2km. At distances shorter than 2km the spot diameter can be larger. Convergent beams are OK.

Also, note that I am not asking how to detect amplitude modulations of a light source that is 2km away. That would be an entirely different question, which would involve myriad of additional considerations such as SNR and the performance of some kind of receiver with its light gathering optics, detectors, etc... This is not what this question is asking about.

Also, I am not asking for a link to a ready-to-buy product - just for an approximate figure based on your experience in the marketplace.
Finally, I would appreciate some general technical advice for achieving the smallest diameter spot at this distance and price, e.g.: what light source to select and what type of optics to use to emit the beam.

Regards,
Pavel Vicek
 
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When you say 200mm at the emitter you mean the output optic, Right ? Yet you say any physical size laser.....interesting.

So you want a high quality beam with a long coherence length run through a beam expander with an output lens dia. of 8 inches or 200mm right ?

So you want to know if a dpss laser or a gas laser has the better beam right ?

Well one way to increase coherence length is to increase the length of the cavity while maintaining the highest possible Q so I would lean towards gas lasers......however when it comes to cavity length per dollar you just can't beat active fiber past the point of readily available glass tube lasers, so I expect just as our military has discovered that fiber is the way to go.

For a 5k budget it would mostly depend on what you can find vs. having anything made ( such as a beam expander ) and will you build it your self or pay someone else.
 

Zoomby

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When you say 200mm at the emitter you mean the output optic, Right ? Yet you say any physical size laser.....interesting.
The 200mm is the beam diameter at the output optics. Obviously, the entire device (including the output optics) is larger and 3 dimensional, thus do describe its size, I would need to give you three separate dimensions, however I am not limited by them, so I just wrote: "Any".

As far as as the light source - I don't have enough information yet to decide whether a laser is the best choice for this purpose (I do not require monochromatic, phase coherent light) so I am not facing a decision yet whether to chose DPSS laser or a gas laser or no laser at all.

For a 5k budget it would mostly depend on what you can find vs. having anything made ( such as a beam expander ) and will you build it your self or pay someone else.
I would be willing to build it out of prefabricated components such as off-the-shelf telecentric lens and laser or LED (just guessing here), but I would not want to go as far as grinding my own lenses or mirrors and pumping vacuum out of glass lasing tubes.

I could pay someone to make e.g. a beam expander but I have no idea how much such services cost and whether it would not be cheaper to buy a prefabricated one.
 
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WizardG

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The 200mm is the beam diameter at the output optics. Obviously, the entire device (including the output optics) is larger and 3 dimensional, thus do describe its size, I would need to give you three separate dimensions, however I am not limited by them, so I just wrote: "Any".

As far as as the light source - I don't have enough information yet to decide whether a laser is the best choice for this purpose (I do not require monochromatic, phase coherent light) so I am not facing a decision yet whether to chose DPSS laser or a gas laser or no laser at all.


I would be willing to build it out of prefabricated components such as off-the-shelf telecentric lens and laser or LED (just guessing here), but I would not want to go as far as grinding my own lenses or mirrors and pumping vacuum out of glass lasing tubes.

I could pay someone to make e.g. a beam expander but I have no idea how much such services cost and whether it would not be cheaper to buy a prefabricated one.
An 8 inch newtonian telescope would do as a beam expander. ~$750 new retail for the optical tube assembly.
 
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The 200mm is the beam diameter at the output optics. Obviously, the entire device (including the output optics) is larger and 3 dimensional, thus do describe its size, I would need to give you three separate dimensions, however I am not limited by them, so I just wrote: "Any".

As far as as the light source - I don't have enough information yet to decide whether a laser is the best choice for this purpose (I do not require monochromatic, phase coherent light) so I am not facing a decision yet whether to chose DPSS laser or a gas laser or no laser at all.


I would be willing to build it out of prefabricated components such as off-the-shelf telecentric lens and laser or LED (just guessing here), but I would not want to go as far as grinding my own lenses or mirrors and pumping vacuum out of glass lasing tubes.

I could pay someone to make e.g. a beam expander but I have no idea how much such services cost and whether it would not be cheaper to buy a prefabricated one.

So is your question academic or do you have an actual task you wish to accomplish ?
 

Zoomby

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An 8 inch newtonian telescope would do as a beam expander. ~$750 new retail for the optical tube assembly.
That uses a parabolic mirror doesn't it? Also, do you mean to input the light at the focal point of that mirror ...or apply the light source to the secondary mirror?
 
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kecked

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Send a regular laser likely opsl or gas 2km and focus the beam to a spot. If all you want is a tiny spot from the source that will do fine. Not very efficient but would work. Also what is the horizon distance. Is 2km beyond that cuz then you need altitude.
 

Zoomby

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Send a regular laser likely opsl or gas 2km and focus the beam to a spot. If all you want is a tiny spot from the source that will do fine.
What optics do you suggest ot focus it to a minimum at 2km distance? ...and what would be the minimum spot diameter with your solution ?

Not very efficient but would work. Also what is the horizon distance. Is 2km beyond that cuz then you need altitude.
At 2km the altitude requirement is trivial. ~16cm.
 

kecked

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You missed my point. You don't need to do much of anything. Add a beam expander on the sending end just to tighten the beam a bit. Than on the receiving end the beam will cover a large area so just grab a sample of the beam and focus it down to a spot. If the spot size on the received side is what is important than that should work. Maybe you are looking to use an optical link for communication and then back into a fiber? 2km is pretty small distance and if you do the math you will find the beam size from say a hene with a 2-4x beam expander isn't all that large. maybe 6-10 feet If all you need is small spot size this should do it. Not much to say about security of the data however.

If i remember a 1-2milliradian divergence gives a spot size around 3-6 feet at a kilometer. cut it down to 0.25Mr and you should have a workable size on the receiving side. Now you could receive this with a crude parabolic mirror and focus it down to preserve power and then focus the collected light once more to a point.

Now i think of it two 8" telescopes could do nicely. One on each end.

this might help you on spot size. https://www.gentec-eo.com/blog/spot-size-of-laser-beam

One think not mentioned is atmospheric effects. Everything i said is thinking like there is a vacuum. Well there isn't so expect the beame to move around. one reason to want a slightly larger beam.
 

Zoomby

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If i remember a 1-2milliradian divergence gives a spot size around 3-6 feet at a kilometer...
...and that would mean that at 2km the spot diameter would be 6-12 feet !

..cut it down to 0.25mr and you should have a workable size on the receiving side.
That would mean 1.5 - 3 feet spot diameter at 2km distance. Is this the best a $5000 can buy ?

Now you could receive this...
Your reply assumes that this is for communications and "receiving" , seeing or detecting the amplitude modulation of the light source from 2km away is the goal. FYI: It is not.

In the title and in my first post of this thread I had clearly stated, that my goal is not to create the lowest divergence beam at that price, but to create the smallest spot diameter at 2km. These are two different goals, and only the latter is mine.

This is the reason why I wrote: "At distances shorter than 2km the spot diameter can be larger."
 
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Over long distances the atmosphere will distort the beam, this is why telescopes use adaptive optics, at 2km I don't know how much effect this will have but I know looking through my telescope at a focused beam a few hundred meters away it's not a round spot anymore, but to the eye at the users end it looks like it's a small spot when actually it's a 10 foot wide splotch.

That said some wavelengths traverse the air better than others.

1670445148223.png
 

kecked

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Well since you never stated the objective I had to guess. Spot size and divergence are two sides of the same coin. You can only keep the spot size down by decreasing divergence unless you refocus at the other end. I did look for curiosity if you can create an f/12000 lens but didn’t see such an animal this you must balance divergence and spot size to get as far away from the source before divergence takes over again. So despite you desires divergence and spot size are both important. Now one possibility which again wastes beam would be to reduce divergence by some large factor which you can easily do and then mask the beam to the size you want. Interesting application I wish you luck. I have nothing more to add.
 
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LOL I was also speculating on the FL of a 200mm wide output lens, here are some calculators.

 
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Something I read long ago. To get a small spot, spread the beam, then focus the beam.
 




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