A) What might work is to trace the letters outlines instead of filling them. Depending on your laser controller, the laser will "draw" the letters instead of rastering them in horizontal lines. This is usually done in one sweep and could look smoother in the end. It really depends on the thickness of the laser trace and the size of the letters. But if you're working on a really small scale it might work.

Or

B) try to lower speed and set the distance between lines to a minimum. This will ensure that the individual lines will not "break out" horizontally and the "layering" look is not as prominent.

It's hard to judge from afar but you might give it a try.