Is there a trick for cutting letters to avoid burnt edges?

The most common material that ends in a burnt edge when laser cutting is woods. To reduce this effect there are a few things that can be done: run much faster with the laser so the laser does not have time to build up heat (high laser powers are recommended). Lower laser wattages can be run with more than one pass to reduce the burnt edge (not recommended with other materials). 

Example: running at twice the speed to cut wood with two passes will take the same time, but each pass will move faster, producing less of a dark look as there is less time to build up the heat in the surrounding material. Another option is to use a wood that is less dense; for example, cutting woods like maple versus pine versus balsa woods – each wood has a totally different density, power, and speed. To cut each wood at the same thickness is totally different; the denser woods like maple produce the darkest look as it is the hardest wood, but pine being a medium density wood, can be cut much faster and, therefore, produce a lighter edge.  Balsa wood will be even better as it is one of the least dense woods.

   —Trotec Laser