The reasons why lasers that produce very short pulses of light (called either ultrafast or ultrashort pulse lasers) generally produce high quality micromachining results can be traced to the mechanisms by which these pulses interact with matter.
Machining with Ultrafast Lasers
Ultrafast pulses of light interact with matter in a manner that is totally different from the way traditional laser pulses interact with materials! Why this happens will be explained in more detail in the following sections, but for now it is important that you let go of the belief that what we are about to describe to you is simply the same micromachining process done on a different time scale.
For now, we can summarize the conclusion by saying that micromachining quality is a strong function of the amount of heat deposited in the work piece, or more exactly, a function of the amount of heat that is left behind in the material that can and does cause damage. Ultrafast pulses are extremely short by any standard. So short that the energy they deposit in material does not have time to leak away from the micromachining spot via mechanisms like thermal conduction. So much energy is deposited in the material so fast that the material is forced into a state of matter that physicists call a plasma. This plasma then expands away from the material as a highly energetic gas, taking almost all the heat away with it. Essentially, the material goes from a solid to a gas phase without first going through a melt phase. Consequently, very little heat is left behind to damage the material. This means that the machining quality is high.
No other kind of machining can create this very highly energetic state of matter. In part it is the unique ability of ultrafast lasers to create this state that is the reason why they produce results so different from those produced by traditional lasers used to machine materials.
But there are other advantages. To find out more, read on...
A more detailed explanation…
The intent of the following presentation is to give you a detailed understanding of the processes involved in traditional laser machining, so that you can better understand how ultrafast laser machining differs from traditional machining techniques.
We should begin by saying that these are complex - which is a physicist's way of saying that some things are not yet fully understood. In spite of this, however, we do know that the very short optical pulses interact with matter in a manner that is totally different from other types of machining, including conventional laser machining. We know, for example, that the interaction of ultrashort laser pulses with matter is highly reproducible (for reasons that will be discussed in Chapter 9). This makes for high quality micromachining with very reproducible results, shot-after-shot.
To make this complex science reasonably understandable, we have simplified or ignored many issues. Should you be interested in a more in-depth reading on the subject, please contact us and we will be happy to provide you with a reference list of relevant research papers.