You need a reel of monofilament wire of nitinol or other smart material (I think fullerenes have shown "smart" properties) that will straighten when subjected to electricity, magnetism, sound, or whatever the catalyst is, mounted in the saber handle. A ring of lasers around the wire will be angled to converge on the end of the wire so you know where it is and avoid the "blade". Instead of lasers, a coherent magnetic field could be used around the wire to contain a noble gas that the wire would excite, like a magnetic fluorescent bulb, though that would need much more power.
When activated, the monofilament would be cast out and straightened by the catalyst, and outlined by the lasers or fluorescent gas. The monofilament should be able to slice through anything, like a light saber, and block another saber's blade like in the movies, depending on the strength of the wire, which would be invisible. The "woom" sound of a light saber might be produced by the catalyst's generator. When deactivated, the wire would be reeled back into the handle. The casting and reeling, with the light outline, would look like the activation and deactivation of a movie saber.
I could have waited until I got a patent before telling how to make them, but I doubt that would happen any time soon. But at least I'm establishing now that I invented it. Or at least, I have foreseen it.