There are a lot of plastic types out there that can be marked with a conventional fiber laser. Most of the time, the marking result is exceptional. However, some plastics cannot achieve a reasonable homogeneity. It is these plastics that can easily be marked with a MOPA laser, resulting in high contrast and quality finishing. The adjustable pulse durations, which can allow either short or long pulses, also help significantly.
Dark plastic (PA 66 GF, PA 6 GF and PP GF) doesn't heat very much due to the short pulses and the associated low pulse energy. This means that the plastic won't foam as much, and the marking will be more consistent. Other kinds of plastic require a greater amount of energy to heat up the material sufficiently. In these cases, the long pulses can help to optimise the foaming. Either way, marking with the MOPA laser creates a clearer contrast and better machine readability.