How to achieve a wide & long beam?

[genebrecht]

Greetings,

I have been reading this forum for about a few months, trying to learn about lasers, specifically about building one (in my case 2). I need to build a laser that will be seen 2 km away when the operator is pointing it up, basically as a signal.

I have seen a few beam expanders and they look like they might do the trick. I wonder though how much less distance will the laser shine with a beam expander. As to the colour it is going to be green but unsure of the specific wavelength and wattage. And this laser will not be used for burning, but i am open to the idea. DX Lean Diet Forskolin

so i have a few questions before I build this;

1. from how far away will a normal beam be visible to the naked eye? (if the operator is poinintg it up, not at the observer)

2. beam expander vs no beam expander - how long is the beam when compared to with and without?

3. how long can a laser shine assuming unlimited battery/power supply?


any reference to specific posts/info will be greatly appreciated , thnx

 

[smallfreak]

1. from how far away will a normal beam be visible to the naked eye? (if the operator is poinintg it up, not at the observer)

It all depends on the atmospheric conditions. Clean, dry air is the worst case. You will only see any light, when it is deflectet toward your eyes. Tiny droplets of moist, dust, smoke and similar. You can't see a "passing" ray of light. The colorful Laser beams in StarWars, that make a great impression in the movie are not possible in space, as there is nothing that may be iluminated in vacuum.

The visibility is best ALONG the beam, either from behind or from the front. Perpendicular viewing conditions are the least preferable. The precise dependency is "complicated", but your application will either need a very strong beam or at least misty conditions. More matter to "cut through" = more visible.

Pointing a strong Laser into the sky is a risky operation. 99.99% of the power might go straight up and maybe just a miniscule fraction of the light energy gets scattered to be watched by the observer. Since you cannot limit the ray to a given length, it will eventually go to the end of the universe, but you may hit something a few hundred meters above you that you didn't think of before.

That's why shooting Laser beams anywhere is usually strictly regulated. You might want to read about your local applicable laws on that matter. You might have to apply for a special permission to do such kind of measurements in a public space (and air space is public space) where the beam might leave the defined boundaries of your property.

Ignoring the regulations can have very nasty consequences. Depending on your location you might even get fined for operating "restricted military weapons" otherwise. Pointing with a "clearly visible" Laser beam is a double edged sword, as anyone easily can spot the source. Your essentially schouting "Here I am!". So watch out who is listening.

 

[steve001]

Not practical unless you have a multi-watt laser.

 

[paul1598419]

It is nearly impossible to see a handheld laser's vertical beam from 2 miles away. It would take a green laser of many watts to even approach what you are trying to accomplish. You might have better luck with a high powered flashlight as it would light up your location at night where someone might be able to see it.

 

[Cyparagon]

I need to build a laser that will be seen 2 km away when the operator is pointing it up, basically as a signal.

Why?

Why do you need a signal, why are you insisting on lasers, why straight up, why 2km? We can help you a lot better if you actually tell us what you're trying to accomplish.

 

[smallfreak]

This is what you are looking for: https://en.wikipedia.org/wiki/Searchlight

The problem with a Laser is its small beam size, even with an expander. You need a big volume of air, containing a big volume of particles that can deflect the light. A tiny, highly colimated and powerful Laser beam will get to narrow to be seen from a distance, even if it is quite bright.

The danger/brightness factor for the BEAM is very unsatisfying with a Laser.

 

[paul1598419]

I thought of search lights, but they are normally quite large and need a lot of power to operate. If you could get something like this effect with a high powered flashlight, it might be the best option of all of them.

 

[Nae K]

You said you needed a signal, smallfreak suggested a search light but I have a flare gun from bass pro I've used a couple of times and can easily be seen from 2km away. Maybe that would work. It would help to know what you're using it for because right tool for right job and all that.

 

[smallfreak]

If you could get something like this effect with a high powered flashlight, it might be the best option of all of them.

To get a good visible beam, you need a couple of cm diameter, so any "strong flashlight" with a 3+W LED might serve as a good starting point.

Maybe you could apply this technology

on a common Flashlight with a Cree XHP70 LED. To achieve wider beam without a bulky lens, one could utilize a fresnel lens instead, while still keeping a reasonable beam profile over long distance.

Not a Laser, but a mighty and probably visible beam.

Or you might try one of these:
https://store.marinebeam.com/marinebeam-ultra-long-range-led-illuminator-flashlight/

 

[Cyparagon]

Maybe you all should hold off on the wild recommendations until OP tells us why something like a phone call or walkie talkie won't work.