Building a real lightsaber would be sick but it would be fairly hard to do. Laser beams are parallel rays of light, they'll go very far at the same intensity. If you put a lens in the way, you can bend the rays of light so that they converge to a focal point. After that the rays start diverging. The intensity will get weaker and weaker the further you go from your focus.
You can get more intensity from the same laser when you converge on a single point rather than simply beam parallel rays. By the same token, if you use what he calls a 'strong' lens, one with a very short focal length, you get a stronger cutting focal point than if the focus converges over a longer distance. The rays of light converge very quickly, then diverge only a short distance behind the target plane.
But a laser that focuses less sharply, in more of a 'tall' X shape rather than a 'short' X shape, will still remain hot along a longer length, though its focal point will never get quite as hot as it would if it was more sharply focused. At the very focus point is where it's going to cut best, but near the focal point it will cut too, as long as you're above the damage threshold of what you're trying to cut. You would have, in effect, a laser with a limited length over which it would be able to cut through objects. Just which objects would depend on how strong your power source was.
Well, it's not quite a lightsaber, but it's a laser blade that can chop someone's head off without burning a hole in the ceiling. There's only a few problems left. One is the issue of reflective surfaces. If you shone the blade into a concave reflective surface, it could refocus at some unknown point and burn something unintended, possibly yourself. When we see our reflection in a spoon, our face appears upside down because it has become refocused by the concave reflective surface.
Then there's the problem of an adequate power supply. As technology goes nowadays, we simply can't cram sufficient power into the handle of a lightsaber to output a searing laser. Nobody would have thought a laser pointer would be possible when the first laser was such a clunky thing, but now we have laser pointers powerful enough to cause damage to someone's eye.
better to burn out...
...then fade away
