Welcome to my first installment on building a lightsaber. This will be the first of a few installments, as I just won't have time to disgorge the whole thing at once. ByTheWay, "lightsaber" is no doubt a trademark of Lucasfilm Ltd. There, I said it.
So what exactly is a lightsaber? In the original script of "The Star Wars", it was called a "lasersword". Way less catchy. The name implies that it is a sword of laser energy-- that is, a laser beam that doesn't go on forever as a ray, but has a limited length like a sword. Tricky.
In the actual "Star Wars" trilogy/sexology (snicker), some random book I read defines a lightsaber as a beam of high-energy particles that goes to the end of the beam, makes a sharp 180, and comes back into the handle. A laser may have been part of the thing. There's also a lot of quasi-mystical stuff about some sort of crystal or gem in the handle. Hmm.
Ok, so how to create the effect? If you just want to know how to create the appearance or sound of a lightsaber in a movie or something, I'm sure you'll find other stuff on the web. If you actually want to cut something, let's talk.
The difficult part of a lightsaber or just plain "lasersword" is the self-termination. (But it's not the most technically challenging part-- more on that later.) Sure, you can just carry a powerful laser around and burn stuff, but it inconveniently keeps going, risking burning and blinding everywhere it's pointed. A lightsaber, on the other hand, can be pointed menacingly at an opponent without punching a hole in them, at least if they are more than a meter away.
The most basic way to do this is to use a mirror or absorbing target at the end of the blade. This would require some sort of solid support, which is patently uncool. Maybe one could fiddle around with magnetic levitation, but that might require holding the lightsaber pretty close to vertical the whole time, cramping a Jedi's fighting style. No doubt you will find a way around this-- superconductive vortex trapping, perhaps?
The next solution is to use a lens that, instead of collumating the beam, focuses it to a point a short distance from the handle-hilt thingy. A cone of diffuse laser light converges as it comes out of the handle, becomes intense at and around the focus, and then diverges afterwards. (Need a picture? Maybe later.) So at a distance from the focus, the beam becomes a lot less dangerous, although still prone to blinding people. To get a nice blade, the cone should converge slowly, elongating the effective focus into a stretched-out ellipse. The problem is that the laser light then diverges slowly out of the focus, making the danger zone very long. As in all engineering, there is a trade-off; a long blade requires slow divergence; a nice cut-off makes the beam inconveniently short.
Is there some way to get around this? Well, since the optics are all in the hilt, it's difficult to change the beam once it's gone out. But here's something that could be done. What if, instead of a single lens, the laser were focused by a series of concentric rings?
Here's the idea. The blade is a dotted line instead of continuous. The innermost ring/lens focuses the laser light to a bright point very close to the hilt. This laser light disperses quickly as one moves away from the hilt. The next ring out focuses laser light into a point just beyond the first. This disperses slightly more slowly as one moves away from the handle-- but this is only a portion of the total laser energy, so it's not as dangerous as focusing the whole beam there.
So continue until you have a series of bright points the length of the blade. Now the laser energy is distributed in a way much closer to our ideal-- evenly along the blade, and much less elsewhere. By making more and more points closer and closer together, in the limit, we get a continuous beam. The Force is now with us!
I haven't done a study of this, but I imagine the optical element to resemble a Fresnel lens, a CD, or some sort of holographic optical element. Actually, it might be a lot more boring than that, just a properly shaped piece of glass.
Hmm, maybe this isn't such an imaginary hack. I wonder...
But what about high-energy particles? More on that later.
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