Fiber issue

Assuming you're using EZcad---

Try a different hatch routine, my bet is your using the blue hatch:
Attachment 350769
- this hatch engraves the sides as shown, and it follows the quickest path, regardless of the direction, for example, it will engrave an "O" up one side, then down the other side. This means there can be visible seams when the path is broken in one place and is joined afterward from a different direction. This may or may not be what's causing your lines.

If you are using the blue, change to this blue/red hatch-
Attachment 350770
this hatch doesn't engrave the sides, indicated by the red in the icon. And more importantly, the hatch runs in one direction only, as in, if it starts engraving in the bottom right corner of an "0", it will continue ONLY diagonally upwards towards the top left corner, covering the entire "0" in one pass. It's slower but the result is a much cleaner finish.

IF you're already using the blue-red hatch, then your laser may have a communication problem, either between the computer and machine, or possibly between the control board and the scanhead...