Laser weapons in space - distance and focal lenses

Assume for a moment that energy-based laser weaponry is the way to go in space.

I am under the impression that, unlike a kinetic weapon, a laser will have a certain distance where the energy output on hit per m² will be at its largest, depending on the lense used.

This would mean that, as the target is moving either closer to the lasers origin or moving away from the lasers origin, as long as the distance from the laser optimal focal point is increasing, the laser will be less effective.

For that purpose, a ship would need to have 3 different types of lasers:

• One for long range distances
• one for medium range distances
• one for short range distances

All 3 type of lasers could be same and have the same energy Output, but would be using a different lense and to have a different focal point per distance.

Is this a correct assumption ?

Would it be (realistictly) possible to construct a laser weapon that is able to shift its lense in a way that it can effectively alter its optimal "range" on the fly as to offer the most energy concentration per m² on 10000km, 100000km and 1000000km distance ?

• Your second paragraph is true. Others not so. I can't write an answer now, but look how zoom camera lens works. Now, you care for only one wavelength, so all chromatic aberrations are not a problem for you. And you only care about one point, so spherical aberrations aren't a problem for you, either... Sep 12, 2016 at 7:11
• Try bessel beam instead. Sep 12, 2016 at 7:51
• You wouldnt need to shift lenses, only swap out the final focusing lense. Jan 11, 2018 at 1:01

Think of focusing a camera lens. Being monochromatic it won’t be so compex and the limiting factor will be how far you can move the elements apart.

Even if there is some engineering practicality, why not simply change lenses (or fit in alternatives for some portion of the lens elements)? It would be just as fast to move it sideways to pop a different one into the beam’s path.

Or, maybe focusing is done with active holograms or changing the spacing of micromechanical ridges. So focus can be near instantanious without moving huge components around, and offers an enourmous range.

But, is that necessary? With a camera you have a depth of field and a point at which the depth of field goes to infinity, the hyperfocal distance. Does this same concept apply in reverse?

@RoryAlsop’s notes would indicate that it does, since atom-sized changes will make a significant difference as you get far enough away.

And you can’t focus to a point anyway! You have conservation of étendue which xkcd illustrates; but with a laser Øin is very close to parallel beam sides, but quantum effects make the beam diverge even if “perfect”.

So realisticly you’re looking at a spot of some size with most intensity at the center and tapering off. If the spot’s core is the size of a quarter, does it matter if it’s a millimeter larger or smaller? So focusing will not matter for movement of the target.

Now if you only have rough focusing lenses (or settings) for different operating distances, you might arrange things so that any meaningful shot is “far”.

If the primary lens/mirror is very large that helps get a tight spot at far distance, but means that what is considered “far” will be farther down range. So you may indeed realistically describe close combat evading the weapon because it can’t focus so close. How close depends on the actual numbers involved.

So yes, optimizing the effectiveness for distant targets will hurt the close-up performance. The huge thing will be slower to turn, too, which is significant close up.

But surprise, the close-in fighters did not expect you to have an active holographic diffractive optic element, which allows you to focus close as well as track the beam faster than a mechanical tourret could be expected to move!

※ I love it when technobabble turns out to be a real thing!

Beyond a certain distance, the focus is effectively at infinity, as your light "rays" become almost parallel.

As an example, if you have a huge 100 metres wide laser beam, using a lens to focus it at a point 100 metres away will give a very strong focus.

At 100 kilometres away, the beam is almost parallel, and at 10,000 kilometres it pretty much is parallel.

Having just run the maths (using my old high school SOH CAH TOA) I get the following:

100m focal length has an angle of 63° from perpendicular to the lens 100km has an angle of 89° 10,000km has an angle of 89.999°

So your lens is not going to be very useful, beyond a very short distance, and in fact minor imperfections will have a huge effect. Imagine accidentally altering your lens to change the incident angle from 89 degrees to 89.9997 degrees (a pretty small change) - that changes your focal length by 9,900km!

• if it is not fiber laser, then using adaptive mirror probably is way generate beam with needed focal point just out of the box. Sep 12, 2016 at 16:30
• that's irrelevant - for that purpose a mirror has the same problems as a lens. Your beam size is just such a small percentage of the focal length that you can't focus. Sep 12, 2016 at 17:03

Your real problem here is that you are mixing your understanding of different types of optics. The camera lens analogy does not apply to a laser.

When you "focus" a laser beam you are not creating a converging focal point upon which the photons converge to a point and then scatter beyond that.

The point of a laser beam is that you want specifically to avoid creating such a focal point at all. You want all of your photons to be travelling as parallel as possible.

The question then becomes one of how you focus (not focus to a focal point) the beam such that the photons run parallel for as far a distance as possible. Whatever the answer you apply to your long range is still the same answer for your short range; that is, if the photons travel parallel out to a long distance then they are already traveling parallel in the short distance. So you do not end up with the problem you are describing and, at least as far as distance is concerned, the device will be one size fits all.

• Side note: "focus photons to run parallel" can be seen to be equivalent to "converging photons with a very distant focal point," but that still amounts to "The long-distance laser still works for close/medium." Mar 20, 2017 at 13:06

The ever useful Atomic Rockets site has a great section on laser weaponry, but the conclusion is far different than what you seem to be implying. Rather than have a multitude of laser weapons or optical systems, the ultimate aim is to create a Ravening Beam of Death (RBoD) and attack targets from as great a distance as possible.

For practical reasons, this turns out to be one light second (just under the distance from the Earth to the Moon), since you can see the target, aim and make corrections in such a short time frame that the target cannot move an appreciable distance. The massive Free Electron Laser (or actually Xaser, since it is fired in the x-ray frequencies) near the end of the section can vaporize metal, ceramic and carbon in milliseconds at that range, and if you are on an unpowered orbit or on an asteroid, the beam is still lethal at a light minute and dangerous even a light hour away.

Let's take a 10 MW ERC pumped FEL at just above the lead K-edge. This particular wavelength is used because lead is pretty much the heaviest non-radioactive element you can get, and at just above the highest core level absorption for a material you can get total external reflection at grazing angles - so no absorption or heating of a lead grazing incidence mirror. We will use a 1 meter diameter mirror. The Pb K-edge x-ray transition radiates at 1.4E-11 m. This gives us a divergence angle of 1.4E-11 radians. At 1 light second, we get a spot size of 5 mm, and an intensity of 5E11 W/m2.

Looking at the NIST table of x-ray attenuation coefficients, and noting that 1.4E-11 m is a 88 keV photon, we find an attenuation coefficient of about 0.5 cm2/g for iron (we'll use this for steel), 0.15 cm2/g for graphite (we'll use this for high tech carbon materials) and 0.18 cm2/g for borosilicate glass (a very rough approximation for ceramics). Since graphite has a density of 1.7 g/cm3, we get a 1/e falloff distance (attenuation length) of 4 cm. Iron, with a density of 7.9 g/cm3, has an attenuation length of 0.25 cm. Glass, density 2.2 g/cm3, has an attenuation length of 2.5 cm.

At 1 light second, therefore, the beam is depositing 2E12 W/cm3 in iron at the surface and 7E11 W/cm3 at 0.25 cm depth; 1.2E11 W/cm3 in graphite at the surface and 5E10 W/cm3 at 4 cm depth; and 2E11 W/cm3 in glass at the surface and 7E10 W/cm3 at 2.5 cm depth. Using 6E4 J/cm3 to vaporize iron initially at 300 K, we find that iron flashes to vapor within a microsecond to a depth of 0.9 cm. The glass, assumed to take 4.5E4 J/cm3 to vaporize (roughly appropriate for quartz) will flash to vapor within a microsecond to a depth of 4 cm within a microsecond. Graphite, at 1E5 J/cm3 for vaporization, will flash to vapor to a depth of 0.7 cm within a microsecond (the laser performs better if we let it dwell on graphite for a bit longer, we get a vaporization depth of 10 cm after ten microseconds).

Net conclusion - ravening death beam at one light second.

Now lets look at one light minute. The beam is now 30 cm across. This is much deeper than the attenuation length in all cases, so we will just find the radiant intensity and the equilibrium black body temperature of that intensity. We have an area of 7E-2 m2, and an intensity of 1.4E8 W/m2. You need to reach 7000 K before the irradiated surface is radiating as much energy away as heat as it is receiving as coherent x-rays. The boiling point of iron is 3023 K, the boiling point of quartz is 2503 K, and the sublimation temperature of graphite is 3640 K. All of these will be vaporized long before they stop gaining heat. At this range, the iron is subject to 5.6E8 W/cm3 at the surface, the graphite to 3.3E7 W/cm3 at the surface, and the glass to 5.6E7 W/cm3 at the surface. Using the above values for energy of vaporization, we get about 0.1 milliseconds before the iron starts to vaporize, 0.8 milliseconds before the glass starts to vaporize, and 3 milliseconds before the graphite begins to vaporize (because of its long attenuation length, once it begins to sublimate, graphite sublimates rapidly to a deep depth, while you essentially have to remove the iron layer by layer).

Net conclusion - still a ravening death beam at one light minute.

What about at one light hour? The beam is 18 meters across. The equilibrium black body temperature is 900 K. This is well below the melting point of most structural materials. Ten megawatts, however, is a lot of ionizing radiation. Any unhardened vehicle will be radiation killed at these ranges.

Obviously, the ideas of "close, medium and far" ranges have very different meanings in a space war context. The only way to effectively deal with a weapon like that is to have several weapons of similar power in your constellation, or be prepared to fill the sky with tens of thousands of kinetic kill vehicles (referred to in Rocketpunk Manifesto as Soda Cans of Death or SCoDs). With an overwhelming number of targets, the individual laser will eventually not be able to track and kill every target, and of course other factors like the service cycle (how often you might have to stop and cool down the system), or the speed the laser mirror can swivel to track incoming targets reduces the absolute number of targets you can service even with a RBoD.

You could try to focus a laser weapon on a single point, but you wouldn't want to. You might want to vary the focus a laser for very close-in work, but at long range you'd want to collimate a laser as well as you can. The presumed benefits of a laser are at those long ranges.

Basically, the problem is that space is big and that nothing is perfect, including how parallel you can collimate your laser.

Imagine that you have a laser projector on your ship, and you fire at another ship. Now, suppose that you want to focus all of your laser's power on a 1 centimeter diameter spot. Modern, high quality lasers can collimate with an error of 1 arcsecond, meaning that they are at most 1 part in 3600 off from perfectly straight. Solving for the trigonometry, at 2 kilometers away, that spot is smaller than your laser's error. 2 kilometers is less than the range of most modern cannons.

So to build on @Rory Alsop's answer, you don't want to focus your laser. You want to collimate it, and accept that the beam will slowly grow over its path, by about a centimeter of error every 2 kilometers of distance.

Changing focus is something our cameras do pretty well nowadays. Having three set of lenses has huge drawbacks comparing with moving one piece:

• risk of collision when you move one in, one out
• mass
• time with no output at all

But don't forget we are shooting lasers on cosmic range now. NASA did it regularly. Few meters wide beam on Earth is four miles wide on the moon. A lot. There are two reasons for this:

• best mirror optics we have now can't do much better
• even if, aiming is really hard

These two should give you a hint what you need to solve or handwave in your story.

If, for some storytelling reason, you want three pieces, consider these:

• Precision. Narrow focus range gives you sharper image, smaller dot. Maybe having three sets is justified by that? (sure it is for me and my photo camera)
• Maintenance. Being able to swap out one of your "muzzles" and still shot is nice.
• Cost. Three simple mirrors are cheaper than zoom optical path.
• Transparency. One mirror reflects more light than one simple lens. And zoom lens are worst.

Realistically, you only need a long range laser. With your design you get a perfect focus in every case. Try replacing it with one big laser with only a long range focus and look what happens:

In almost every engagement you put three times the power on target. In the few where there's a bit of imperfect focus you're still pretty close and probably do more damage than if you were firing your perfect but weaker laser.