Machining
With Long Pulse Lasers
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.
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.
