how to achieve a wide beam? how to achieve a long beam?

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.

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