Laser pulse length (ps)
a) Cutting rate (mm/s)
Repetition rate (kHz)
b) Cutting rate (mm/s)
3.0 2. 5 2.0
E = 75µJ
P = 5W
P = 3W
P = 0.5W
E = 50µJ
E = 25µJ
1. 5 1.0 30
500 400 300 200 100 0
LASER TECHNOLOGY APPLIES TO
SEVERAL MATERIAL TYPES
VYTAUTAS BUTKUS, and SIMAS BUTKUS
Lasers are indispensable tools in many areas of industry, science, and medi- cine. Now, ultrafast laser-based micro- machining is a widespread example of laser source utilization, leading to unprecedentedmicrofabrication quality. Advantages of femtosecond laser
processing over regular mechanical and laser techniques in
automotive, medical instrumentation, and electronics industries
include (but are not limited to) high speed, precision, quality,
and reproducibility, allowing for simplified or, in many cases,
no post-processing operations. This field of laser technology
is currently under intensive development and many exciting
micromachining techniques are yet to emerge.
In laser-assisted manufacturing, every processed material
is a special case and some research is always required. Laser
parameters, such as pulse length, wavelength, repetition rate,
and energy, might determine the processing rate and qual-
ity. Therefore, finding that optimal set of laser parameters and
choosing the right cost-effective technology is very import-
ant. At this point, a highly versatile industrial laser with tunable
pulse length, energy, and repetition rate becomes a vital part
of the whole system.
In one way or another, the desired mechanical modifications
in bulk material or a surface are the results of related physical
phenomena, occurring right after deposition of light energy
to a material. In the simplest terms, these processes can be
divided into “hot” and “cold” micromachining.
In hot micromachining, solid phase material at the optical
pulse-material interaction zone is transformed into the gas
phase from local heating, leading to vaporization and ejection
of material. Local heating is achieved because of the elec-
tron-phonon interaction of the excited free electrons and the
material crystal lattice.
In hot ablation (i.e., material removal), laser pulse length and
material removal rate is comparable to the timescales of mate-
rial thermal diffusion. As a result, heat is transferred outside
the ablation zone and might cause collateral thermal damage
commonly referred to as heat-affected zone (HAZ). Moreover,
the hot-ablation threshold is very dependent on the number
of material defects and impurities in the ablation zone, leading
to less-precise and less-reproducible micromachining results.
Together, HAZ and ablation variability impede success of some
Detrimental effects because of HAZ and material impurities
are, in theory, completely absent in the working conditions of
cold ablation, offered by femtosecond laser pulses through a
nonlinear absorption process. Since the peak energy of femtosecond pulses is much higher than picosecond or nanosecond pulses, electrons in the valence band are excited to a great
extent, resulting in local material ionization. Enormous electrostatic forces between the ions are established, and the material goes through irreversible changes as it is locally exploded
or cracks are formed. As the process is faster than thermal-ization time, locally induced heat, if any, is effectively stripped
away along with the removed material.
Femtosecond lasers may also offer faster micromachining.
For example, measured cutting rate dependence on laser pulse
FIGURE 1. 50µm-thick Invar foil cutting rate dependence on
laser pulse duration (a) and repetition rate (b) from use of a
1030mm femtosecond PHAROS laser.