Just as CW and quasi-CW lasers have revolutionized the materials processing world, picosecond lasers are poised to
change the world of micromachining, where lasers outperform mechanical tools due to their flexibility, reliability,
reproducibility, ease of programming, and lack of mechanical force or contamination to the part.
Picosecond lasers are established as powerful tools for micromachining. Industrial processes like micro drilling, surface
structuring and thin film ablation benefit from a process, which provides highest precision and minimal thermal impact
for all materials. Applications such as microelectronics, semiconductor, and photovoltaic industries use picosecond lasers
for maximum quality, flexibility, and cost efficiency. The range of parts, manufactured with ps lasers spans from
microscopic diamond tools over large printing cylinders with square feet of structured surface. Cutting glass for display
and PV is a large application, as well. With a smart distribution of energy into groups of ps-pulses at ns-scale separation
(known as burst mode) ablation rates can be increased by one order of magnitude or more for some materials, also
providing a better surface quality under certain conditions. The paper reports on the latest results of the laser technology,
scaling of ablation rates, and various applications in ps-laser micromachining.
We report on a Nd:YVO4 regenerative amplifier (RA), end pumped by 888 nm-diode lasers. The output power
was about 46W at repetition rates from 150 to 833kHz with an M2-factor of 1.2. The amplifier was seeded by
a gain switched diode laser, generating pulses with a duration of 65 ps and a pulse energy of ≈ 5 pJ. The high
gain of the RA of more than 70 dB provides amplified pulse energies as high as 180μJ. Bifurcations of the pulse
energy could be avoided. Pulse amplitude fluctuations of only 1.2% for 10,000 consecutive pulses were measured.
The long term output power stability of the laboratory setup was 0.3%.
High-precision micromachining with picosecond lasers became an established process. Power scaling led to industrial
lasers, generating average power levels well above 50 W for applications like structuring turbine blades, micro moulds,
and solar cells. In this paper we report, how a smart distribution of energy into groups of pulses can significantly improve
ablation rates for some materials, also providing a better surface quality. Machining micro moulds in stainless steel, a net
ablation rate of ~1 mm3/min is routinely achieved, e.g. using pulse energy of 200 μJ at a repetition rate of 200 kHz. This
is industrial standard, and demonstrates an improvement by two orders of magnitude over the recent years.
When the energy was distributed to a burst of 10 pulses (25 μJ), repeated with 200 kHz, the ablation rate of stainless steel
was 5 times higher with the same 50 W average power. Bursts of 10 pulses repeated with 1 MHz (5 μJ) even resulted in
an ablation rate as high as 12 mm3/min. In addition, optimized pulse delays achieved a reduction of the surface roughness
by one order of magnitude, providing Ra values as low as 200 nm. Similar results were performed machining silicon,
scaling the ablation rate from 1.2 mm3/min (1 pulse, 250 μJ, 200 kHz) to 15 mm3/min (6 pulses, 8 μJ, 1 MHz). Burst
machining of ceramics, copper and glass did not change ablation rates, only improved surface quality. For glass
machining, we achieved record-high ablation rates of >50 mm3/min, using a new state-of-the-art laser which could
generate >70 W of average power and repetition rates as high as 2 MHz.
Highest precision and minimal thermal impact is achieved, using picosecond laser pulses for micromachining. Virtually
any material can be processed with outstanding quality. The use of ultraviolet (UV) pulses can provide additional
benefits in higher throughput and improved edge quality for materials like metals, glasses, semiconductors or ceramics.
The advanced oscillator-amplifier (MOPA) laser design, based on reliable Nd:YVO4-systems, enables power scaling of
IR-pulses, and led to a series of laser systems with a repetition rate as high as 1 MHz and average power levels ranging
from 10 W to well above 50 W. Harmonic generation to the visible (532 nm) and the UV (355 nm) reaches efficiencies
of 50%, providing powerful beams for cost effective high-speed micromachining with high throughput.
Nd:YVO4 is a widely used gain medium in commercial lasers providing up to several tens of watts in a diffraction
limited beam. Its high gain favors high repetition rates and short pulses in nanosecond Q-switched and picosecond
mode-locked regimes. However, output power is limited by strong thermo-optical effects leading to an aberrated
thermal lens and ultimately the crystal's fracture. In this contribution, we present the optimized pumping of
vanadate at 888 nm, benefiting from polarization-independent absorption, reduced quantum defect and very low
absorption coefficients compared to the common pump wavelengths of 808 and 880 nm. After a presentation of
the principle and the key characteristics of a high-power fiber-coupled end-pumped multimode oscillator, a series
of systems based on this pumping technique are presented. A compact 60W high-efficiency TEM00 CW oscillator
first proves the potential for high-power high-beam-quality systems. A CW intracavity-doubled system provided
62 W of power at 532 nm. A cavity-dumped Q-switched oscillator providing up to 47 W of average power with
6 ns long pulses at all repetition rates was investigated. Passive mode-locking of an oscillator providing 56 W of
output power was achieved with a saturable absorber mirror. Finally, a high-power oscillator was amplified with
high efficiency in a power amplifier based on the same pump/crystal configuration. The wide range of systems
demonstrated illustrates the simplicity and flexibility of 888 nm pumping for extending the benefits of vanadate
in the higher power range.
Novel solid-state picosecond lasers provide a strong benefit for high precision micro-machining. Pulse repetition rates
as high as > 500 kHz with pulse energies of > 4 μJ enable fast machining with the precision of low fluence ablation.
In addition, new potentials for these lasers are given by advanced modulators with digital timing control that allow the
user to generate sequences or groups of pulses: E.g. a sequence of two pulses can be generated and repeated up to
300 kHz. The amplitude of these two pulses can be adjusted independently and the delay is selectable in 20 ns steps.
This kind of pulse-strategies with picosecond lasers can support higher ablation rates, similar to the machining results
that were demonstrated with double ns-pulses, recently. In another application, groups of > 20 pulses were repeated with
> 50 kHz for ultra-precise machining. The distribution of the energy yields a few hundred nJ per pulse and results in an
ablation depth per pulse in the range of several nm. Therefore the ablation depth formed by a group can be digitally
controlled by the number of pulses in group. Samples for high quality drilling, cutting and structuring of several
materials will be presented and the new potentials of this kind of picosecond laser processing with improved precision
and speed will be discussed.
Laser micromachining is mostly based, so far, on Q-switched laser sources. Their nanosecond pulse width often limits the accuracy and quality of laser processes by thermally initiated effects. Precision micromachining benefits from ultra short laser pulses. Up to now mostly amplified fs lasers with low repetition rates were used, with the result of low processing speed. New diode pumped solid-state picosecond lasers can also meet the demands of precise micro-machining. Their pulse duration of about 10 picoseconds provide the optimum performance e.g. for metal processing. These lasers also provide high average powers and much higher repetition rates of more then 100 kHz to maximize throughput. New potentials of picosecond lasers for the processing of different materials with high precision and increased speed will be discussed.