Machining
materials with lasers - a technology first introduced in the
early 1970's - is now used routinely in many industries. Laser
micromachining is a more recent development. First demonstrated
in the 1980's, micromachining with lasers is an evolving technology.
Initially laser micromachining was based on continuous wave
or long-pulse lasers. With these "conventional"
lasers, the heat transferred from the laser beam to the work
piece introduced numerous restrictions that limit the precision
and the quality of the machining process. In other words,
laser micromachining is... well, not so micro, but rather
course by some of today's standards. Machinists have learned
ways to minimize the negative effects associated with heat
transfer through various types of pre- and post-processing.
These additional steps considerably increase the complexity
and cost of the machining operation.
Note
that heat-diffusion is not limited to laser machining. Tool
bits deposit mechanical energy into the material that is being
machined, a portion of which is converted to heat. This heat
energy does not stay localized where it was initially deposited.
It moves away in a characteristic time - the so-called "heat-diffusion
time." This is a familiar phenomenon. If you turn on
the heating element on an electric stove, it will take a few
seconds to warm up. The same happens at the microscopic level,
but the time scales involved are quite different. The typical
"heat-diffusion time" encountered in laser machining
is not counted in seconds, but rather in picoseconds (a picosecond
is a millionth of a millionth of a second!)
In the
early nineties, scientists at the University of Michigan discovered
that the transfer of heat from the laser beam to the work
piece could be defeated using ultrafast laser pulses instead
of standard long-pulse lasers. Essentially machining with
laser pulses of very short duration eliminates heat flow to
surrounding materials. This discovery opened the way for fine
laser micromachining.
Before
looking in detail at some samples that were machined with
ultrafast pulses, let's first take a closer look at the way
lasers interact with matter. To make this complex science
reasonably understandable, we have simplified or ignored many
issues: we arbitrarily divide the physics of how light interacts
with materials into two time regimes - one in which the laser
pulse is either very, very short (called ultrafast or ultrashort),
and another in which the laser pulse is not so short (which
we call "long"). Ultrafast, or ultrashort, means
that the laser pulse has a duration that is somewhat less
that about 10 picoseconds - usually some fraction of a picosecond
(femtosecond). "Long" means that the pulse is longer
that about 10 picoseconds, that is, longer than the heat-diffusion
time. These long pulse lasers may be continuous, quasi-continuous,
or Q-switched, but in any case they are generating long pulses
compared to the heat-diffusion time.
