How Do Long Range Space Combat Lasers Work?

[Spacescifi]

It is my understanding that the bigger the radius of your laser lens, the greater range you get from a given beam.

Also I have read that lasers are great at focusing all their energy on small spots.

 

So ideally you would want a large radius laser lens to have it's beam converge onto a spot size much smaller than it's radius... right?

And with distance all laser beams widen out anyway.

Which I don't have any issue understanding.

Main question: How would or would a long range laser focus on a tiny spot much smalker than the lens radius? Or would it simply fire a beam equal to the kens radius which grows wider with distance?

The weird thing is that if you really do have a converging long range laser beam which widens out again farther out, there is a sweet spot distance where the beam would miss you if you were not dead center or close to the focal point of the beam.

Since it would be like narrowly avoiding a converging cone shaped beam.

 

Thoughts?

[magnemoe]

It works like an magnifying glass  
And yes past the focus point the beam will widen again, but you know your range because laser rangefinders 

[farmerben]

The big problem in the atmosphere is that high power lasers super heat the air, turning air into a diverging lens.  In space, lasers will be much more effective.  You still face divergence no matter what.

[darthgently]

4 hours ago, farmerben said:

The big problem in the atmosphere is that high power lasers super heat the air, turning air into a diverging lens.  In space, lasers will be much more effective.  You still face divergence no matter what.

Weapon lasers are problematic at distance as you note, but with enough relays sprinkled around, comms lasers could overcome the divergence issues and provide the DSN with fantastic bandwidth 

[kerbiloid]

The book from SDI times with formulas was operating with ~1000 km distance to intercept a protected warhead with an orbital laser.

The focus doesn't play role, as it's anyway much closer to the laser than the target.
Only wavelength/diameter of mirror plays a role.

The orbital lasers use mirrors, not lenses.

[Kerbart]

It looks to me that the problem is mainly precision. If you have the ability to focus a laser beam so it converges to a single point at 1000 km, you should also have the ability to focus it in such a way that it is perfectly parallel and doesn't converge at all. Then it's just a matter of projecting that narrow beam wherever you want. The optical systems required should be relatively simple; the main reasons refractors tend to have complex elements is because they need to be consistent through all wavelengths and with laser light you don't have that issue.

Likely the precision needed for that is beyond our technical capacities. That doesn't mean we can't do it though; feedback mechanisms can correct things but now you have to observe the target and adjust focusing on the fly. In a combat scenario I can see challenges with that.

Then of course there's the energy question. Putting this on a space ship will be a challenge as you'll need a serious powerplant to feed it.

[farmerben]

1000km is short range for space lasers.  The divergence is more an issue at astronomical units.

[StrandedonEarth]

I’m guessing the main issue that even a laser beam will diverge, losing power intensity as it does. A larger beam will still diverge, but power intensity will not drop off as fast with distance as a narrow beam of the same total power. As long as the power intensity (W/m2) is still enough to achieve what it is intended to do

[Spacescifi]

8 hours ago, kerbiloid said:

The book from SDI times with formulas was operating with ~1000 km distance to intercept a protected warhead with an orbital laser.

The focus doesn't play role, as it's anyway much closer to the laser than the target.
Only wavelength/diameter of mirror plays a role.

The orbital lasers use mirrors, not lenses.

 

What? How would that even look? No lens on  laser? I always thought lasers would look like camera lenses, only instead of snapping pictures they burn you with concentrated photonic beams.

And if you can go without a lens then why do atmospheric military shipboard lasers seem to use lenses?

[kerbiloid]

It uses mirrors.

The lenses are on small lasers, in addition to the mirrors.

[Nuke]

bigger aperture longer range i think. max range is limited by optics, but also light delay vs target maneuverability.  at some point the time of light delay exceeds the time it takes for the target clear the beam path at a random angle (thus defeating predictions of where it may be).

lenses are heavy and even very transparent mirrors lenses heat up fast when powerful lasers are going through them. deforming the lens and messing up focus. why i used the more general term optics. mirrors are easier to actively cool as you can have coolant channels baked into the mirror substrate.

Edited March 27 by Nuke

[magnemoe]

8 hours ago, Nuke said:

bigger aperture longer range i think. max range is limited by optics, but also light delay vs target maneuverability.  at some point the time of light delay exceeds the time it takes for the target clear the beam path at a random angle (thus defeating predictions of where it may be).

lenses are heavy and even very transparent mirrors lenses heat up fast when powerful lasers are going through them. deforming the lens and messing up focus. why i used the more general term optics. mirrors are easier to actively cool as you can have coolant channels baked into the mirror substrate.

This, note same is true for naval gunfire, here ship can random walk the shells so guns on say an Iowa is not usable against other warships at their maximum range, lots of earlier battleships could not elevate their gun much but 45 degree is nice for shore bombardment. 
Tanks can also not elevate their guns much, self propelled artillery has much larger turret to let them elevate. 
With lasers you can launch mirrors to bounce and refocus the laser beam. Probably not the best against an warship but against an missile swarm you can start hitting them farther out.