Investigations on laser micro ablation of steel have been made using multipulses instead of single pulses. The aim is to improve the efficiency of laser micro ablation by increasing the ablation rate and quality at the same time. For the improvement of the multipulse laser ablation a flash-lamp pumped Nd:YAG laser with 20 to 100 ns pulse duration is used which can supply multipulses with a specially triggered pockels cell at 10 Hz. A mayor increase of ablation by distribution of energy on several pulses is observed. Further investigations have been made with a commercial ps laser system with a pulse duration of 12 ps at a repetition rate of 100 kHz. The used bursts consisted of up to four pulses with interpulse separations Δt between 20 ns and 100 µs. A cut-off frequency ν cut-off up to which the double pulse ablation equals single pulse ablation is defined by plasma enhanced absorption of the second laser pulse. For the comparison of multipulse and single pulse ablation the geometry and the roughness of the irradiated area have been detected by white light interference microscopy

Arnold Gillner

Claudia Anna Hartmann

Mihael Brajdic

Torsten Fehr

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JLMN-Journal of Laser Micro/Nanoengineering Vol. 2, No. 1, 2007 
Investigation on Laser Micro Ablation of Steel Using Short  
and Ultrashort IR Multipulses 
Claudia Anna HARTMANN*, Torsten FEHR*, Mihael BRAJDIC** and Arnold GILLNER*
* Fraunhofer Institut für Lasertechnik, Steinbachstrasse 15, 52074 Aachen, Germany 
E-mail: claudia.hartmann@ilt.fraunhofer.de 
** Lehrstuhl für Lasertechnik der RWTH Aachen , Steinbachstrasse 15, 52074 Aachen, Germany 
Investigations on laser micro ablation of steel have been made using multipulses instead of 
single pulses. The aim is to improve the efficiency of laser micro ablation by increasing the ablation 
rate and quality at the same time. For the improvement of the multipulse laser ablation a flash-lamp 
pumped Nd:YAG laser with 20 to 100 ns pulse duration is used which can supply multipulses with a 
specially triggered pockels cell at 10 Hz. A mayor increase of ablation by distribution of energy on 
several pulses is observed. Further investigations have been made with a commercial ps laser 
system with a pulse duration of 12 ps at a repetition rate of 100 kHz. The used bursts consisted of up 
to four pulses with interpulse separations Δt between 20 ns and 100 µs. A cut-off frequency νcut-off 
up to which the double pulse ablation equals single pulse ablation is defined by plasma enhanced 
absorption of the second laser pulse. For the comparison of multipulse and single pulse ablation the 
geometry and the roughness of the irradiated area have been detected by white light interference 
microscopy. 
Keywords: laser ablation, micromachining, multipulses, ns pulse burst, ps pulse burst 
1. Introduction  
The use of laser pulse bursts instead of single pulses for 
laser ablation has been investigated by a number of 
researchers mainly for increasing the efficiency of drilling 
of different materials with ultra short pulses [1-3], but also 
with ns up to ms pulses [4-9]. These investigations have 
been carried out either to drill deep holes through the 
sample or to investigate single shot ablation. In our work 
pulse bursts were used for 3 dimensional surface ablation 
of samples of the steel grades 1.4301 (Stainless Steel 304) 
and C75 using ns- and ps- laser radiation with single up to 
four pulse bursts. 
 
2. Experimental setup 
For the experiments with ps and ns pulse bursts two 
different setups have been used. 
 
2.1 Setup for machining with ns pulse bursts 
The experiments with ns pulse bursts were carried out 
with a flash-lamp pumped Nd:YAG laser (λ = 1064 nm, 
HY1200, Lumonics) that generates several pulses within 
one flash-lamp discharge (called pulse bursts) at a 
repetition rate ν = 10 Hz. The pulse duration in single pulse 
operation was 20 ns and increases up to 80 ns for multi-
pulse mode. The interpulse separation Δt between the 
pulses in a bursts can be set between 0.3 µs and 100 µs, 
while the number of pulses per burst can be varied between 
1 and 4. The focus diameter was measured to 55 µm. 
The burst energy EB, defined as the sum of the pulse 
energies within one pulse burst, was kept constant during 
the investigations. Within one burst the energy is 
distributed uniformly among the pulses. 
B
The experiments were carried out in stainless steel 
(1.4301). The sample surface, cleaned with alcohol before 
treatment, was a technical surface. The ablated geometry 
for the experiments was a groove with an overall length of 
3 to 4 mm. The laser beam was focused on the surface of 
the sample which is moved by linear stages with constant 
velocity of 4 respectively 10 mm/min to generate the 
groves. The irradiated laser energy per millimeter 
translation of the sample was kept constant, during the test 
two different values were used.  
 
2.2 Setup for machining with ps pulse bursts 
The experiments with ps pulse bursts were carried out 
with a diode pumped Nd:YVO4 MOPA (Master Oscillator 
Power Amplifier, Rapid, LumeraLaser) laser with two 
amplification stages (λ = 1064 nm). The laser operated at a 
repetition rate  ν = 100 kHz and at a pulse duration of 12 ps. 
Due to the special design of the implemented pockels 
cell well defined pulse bursts could be generated. The 
interpulse separation Δt could be set to n-times 20 ns. 
Experiments were carried out with single and double pulses, 
the burst energy EB in the pulse burst was distributed 
equally on both pulses in the burst. 
B
For the ablation experiments the laser radiation was 
positioned on the surface of the steel C75 by a 
galvanometer scanner system. The ablated geometry were 
pockets, generated by ablating meandrian squares at a scan 
speed of 200 mm/s per line, a line distance of 10 µm 
(overlap ca. 70 %) and 200 layers. 
 
2.3 Diagnostics 
For the determination of the ablation results different 
methods were used: The depth, width and cross section of 
the groves, generated during the experiments with ns pulse 
bursts, were either measured by white light interference 
44 
JLMN-Journal of Laser Micro/Nanoengineering Vol. 2, No. 1, 2007 
microscopy (WIM) (Newview 200, LygoLOT) of the 
surface or by light microscopy of the cross section. 
The depth of the ablated squares were measured with a 
tactile technology after ablation of every 10 layers. The 
roughness of the surfaces is measured with WIM. Before 
measurement of the roughness the samples were etched 
with HCl (30%). 
 
3. Results and discussion 
When two laser pulses with short interpulse separation 
Δt are used for ablation, different processes can influence 
the interaction of the second pulse with the surface of the 
material. The radiation of the first pulse is absorbed by the 
sample, material is removed and a plasma is generated. At 
high pulse energies and high vapourization rates the second 
pulse can be absorbed in the plasma without interacting 
with the surface. At lower energies the plasma generated by 
the second pulse can expand with larger velocity away 
from the surface due to atmospheric preconditioning from 
the first pulse. Plasma assisted ablation can occur resulting 
in an increase of the ablation and/or a decrease of 
roughness. Also the interaction area can be preconditioned, 
so that the second laser pulse can ablate more effectively. 
This preconditioning can be both changes in physical and 
optical properties of the surface (preconditioning of the 
interaction area) and change of the properties of the 
atmosphere and of the ablated material (atmospheric 
preconditioning). Which of these possible processes play a 
dominate role has to be examined. 
 
3.1 Machining with ns pulse bursts 
Using ns-Laser pulses the ablation depth is significantly 
increased by distributing the burst energy EB of the laser 
pulse into several pulses of lower energy within one pulse-
burst. In  the ablation depth in steel for pulse bursts 
with one to four pulses per burst with a cumulated pulse 
energy of E
B
Fig. 1
BB = 2 mJ for two different translation velocities 
is shown. For smaller translation velocities more pulses are 
applied on the same area and therefore the ablated grooves 
are deeper. Due to distribution of the energy to several 
pulses in the burst the ablation depth can be increased such 
significantly, that the melt cannot be removed and therefore 
the grooves are nearly filled with melt. Therefore no 
measurements with four pulse bursts at 4 mm/min are 
possible, see Fig. 1. Three different reasons are possible for 
that increase in depth of the ablation: 
 
• Changing from single pulse mode to multipulse mode 
the pulse duration of the laser radiation increases for 
increasing number of pulses from 14 ns (single pulse) 
to 80 ns (triple pulse). The greater pulse duration 
changes the conditions for melting and vaporisation, 
e.g. more melt and less vapour is generated. 
• More optical energy is absorbed in the sample by less 
absorption of the laser pulses in the plasma due to 
splitting of the energy. 
• Change of conditions for ablation due to pre-
conditioning of the interaction volume, as mentioned 
in paragraph 3, by short interpulse separation Δt. 
 
 
Fig. 1 Ablation depth in steel (1.4301) for single to four pulse 
bursts at same total energy for two different ablation 
velocities (λ = 1064 nm, τ = 20 to 80 ns, ns pulse bursts, 
EB = 2 mJ, ν = 10 Hz). B
 
 
The dependence of the ablation depth from the pulse 
energy and pulse duration was investigated for single pulse 
ablation of craters generated with one laser pulse in steel 
and aluminium. For the pulse durations of concern (20 to 
100 ns) the ablation depth increases with increasing pulse 
duration at same pulse energy (Fig. 2). A possible reason 
for the increase of the depth is a more effective removal of 
the melt with greater pulse durations and less absorption of 
the laser radiation in the plasma which results in more 
optical energy for heating the metal. The ablation depth 
increases with increasing energy for each pulse duration 
and material. 
 
 
Fig. 2 Ablation depth for single pulses craters in aluminium and 
steel as a function of the pulse energy for two different 
pulse durations (λ = 1064 nm). 
 
 
To identify the dependence of the ablation depth from 
distribution of the energy in several pulses the following 
experiment was carried out: the ablation by multipulses in a 
burst is compared to the ablation caused by a series of 
45 
 
JLMN-Journal of Laser Micro/Nanoengineering Vol. 2, No. 1, 2007 
 
single pulses having the same pulse energy as a single 
pulse within the burst and applied as often as the number of 
pulses in the burst [9]. The depth for multipulse ablation is 
increased up to 30 % compared to the ablation depth by 
series of single pulses. The increase of the ablated volume 
due to multipulses is up to 70 %. 
A variation of the interpulse separation Δt of a double 
pulse burst in the range of 300 ns to 80 µs shows that the 
ablation efficiency gets greater with smaller interpulse 
separation Δt [9]. It is shown that due to variation of 
interpulse separation Δt the ablation depth can be increased 
about 20 % for small interpulse separations Δt compared to 
large interpulse separations Δt. The ablated volume can be 
increased by 35 %. During the ablation with pulse bursts a 
cumulative heating of the sample during the burst is 
expected, resulting in an increase of melting and ablation. 
At the ablation of steel with multipulses the depth of 
ablation can be increased by a factor of 10. This 
improvement of ablation can be ascribed as shown above to 
the increase in pulse duration, the distribution of energy on 
several pulses and the small interpulse separation Δt with 
subsequent preconditioning of the sample. 
For the measured increase in the ablation depth with ns 
pulse bursts, the most dominant effect is the distribution of 
the energy on several pulses. This can be explained by a 
reduced self-absorption of the laser radiation in the plasma 
formed during ablation. The depth gets greater with greater 
number of pulses per burst. 
The ablation depth increases with larger pulse durations 
by a factor of ca. 2. This can be explained due to less self-
absorption of the laser pulse in the plasma as the plasma 
density gets smaller and therefore the optical energy 
decreases that is absorbed in the plasma. Also the melt is 
removed more effectively due to the larger heating time of 
the plasma that expels the melt. 
By decreasing the interpulse separation Δt the ablation 
depth with double pulses increases about 20 %. This 
increase can be explained due to cumulative heating of the 
surface during the pulse burst. This heating results in a 
bigger amount of melt that gets expelled more effectively 
by the following pulses in the burst. Also due to the pre-
conditioning of the interaction volume (paragraph 3) less 
energy is needed for vaporisation and expulsion of plasma 
as there is a reduction of the density in atmosphere close to 
the sample surface. Additional the ejecta e.g. droplets of the 
first pulse get heated and are more effectively removed 
from the surface [7]. 
 
3.2 Machining with ps pulse bursts 
For micromachining with ps-Lasers the ablation depth 
per layer is investigated for double pulses as a function of 
the interpulse separation Δt. For small burst energies 
(EB = 4 µJ, ) the distribution of the energy into two 
pulses increases the ablation depth only slightly. For 
smaller energies than E
B Fig. 3
BB = 4 µJ the ablation depth is even 
smaller for double pulses than for single pulses of same 
total energy as the energy of the pulses in the burst gets 
close to the ablation threshold. 
For machining with larger burst energy (EB = 22 µJ, 
) distributing the energy into pulse burst increases the 
ablation depth by about 50%. The comparison of ablation 
depth for double pulses at a burst energy E
B Fig. 
4
BB with two times 
the depth of the single pulse ablation at the energy EB/2 
(which means the same applied energy per area) shows, 
that by using pulse bursts the ablation depth can only 
slightly be increased up to 10 %. The ablation depth 
approaches a constant value for larger interpulse 
separations Δt. At large burst energies and small interpulse 
separation Δt < 100 ns the ablation depth with double 
pulses decreases with decreasing interpulse separation Δt 
and gets even smaller than two times the ablation depth 
with single pulses of half burst energy E
B
BB/2. 
As shown in Fig. 3 and Fig. 4 the ablation depth with 
double pulses can not significantly be increased compared 
to twice the ablation depth with single pulses of the half 
burst energy EB/2. It only is grater when compared to the 
depth machined with single pulses of same burst energy E
B
BB. 
 
 
Fig. 3 Ablation depth in steel (C75) with 4 µJ ps-double pulse 
bursts as a function of the interpulse separation Δt 
(λ = 1064 nm, τ = 12 ps, EB = 4 µJ, ν = 100 kHz). B
 
 
Fig. 4 Ablation depth in steel (C75) with 22 µJ ps-double pulse 
bursts as a function of the interpulse separation Δt 
(λ = 1064 nm, τ = 12 ps, EB = 22 µJ, ν = 100 kHz). B
 
 
46 
JLMN-Journal of Laser Micro/Nanoengineering Vol. 2, No. 1, 2007 
Evaluating the roughness of the ablated areas 
depending on the interpulse separation Δt results in curves 
which can be divided into three sections (Fig. 5 and Fig. 6): 
• Section I: For 0 ns < Δt < 40 ns the roughness is 
smaller than for larger interpulse separation Δt. 
• Section II: For 40 ns < Δt < 400 to 700 ns the 
roughness is much larger than for any other interpulse 
separation Δt, but gets smaller with greater interpulse 
separation Δt. 
• Section III: For Δt > 400 to 700 ns the roughness gets 
smaller than in the first section and is in the range of 
the roughness for ablation with single pulses at half 
burst energy EB. B
 
 
Fig. 5 Roughness Ra of the ablated area as a function of the 
interpulse separation Δt generated with 4 µJ ps-double 
pulse bursts (λ = 1064 nm, τ = 12 ps, EB = 4 µJ, 
ν = 100 kHz). 
B
 
 
Fig. 6 Roughness Ra of the ablated area as a function of the 
interpulse separation Δt generated with 22 µJ ps-double 
pulse bursts (λ = 1064 nm, τ = 12 ps, EB = 22 µJ, 
ν = 100 kHz). 
B
 
 
Comparing the roughness and the ablation depth the 
three sections can be explained as follows: 
Section I 
At very small interpulse separations 0 ns < Δt < 40 ns a 
strong absorption of the second pulse in the plasma is 
expected. Less optical energy of the second pulse is 
transmitted through to the plasma to the sample, the plasma 
itself remelts the surface and assists the ablation and 
decreases roughness. 
 
Section II 
Setting the interpulse separation between 40 ns < Δt < 
400 to 700 ns influences the ablation negatively. The 
ablation depth increases with greater interpulse separation 
Δt till it gets constant. At the same time the roughness is 
increased, too, and decreases with greater interpulse 
separation Δt. A high-energy plasma is generated remelting 
the surface explosively due to the interaction of the second 
laser pulse with the plasma. The larger the interpulse 
separation Δt is the smaller becomes the influence of the 
plasma and the roughness. 
 
Section III 
Increasing the interpulse separation Δt > 400 to 700 ns 
the probability of interaction of the second pulse with the 
plasma generated by the first pulse gets significantly 
smaller. The ablation depth with double pulses equals to 
twice ablation depth with single pulses of EB/2. The 
threshold interpulse separation Δt
B
tr defines a cut-off 
frequency νcut-off: for ν < νcut-off the ablation with double 
pulses equals to single pulse ablation with half burst energy. 
This means that the plasma generated by the first pulse has 
no detectable influence on the second pulse. The thus 
defined cut-off frequency νcut-off ranges between 1 and 
3 MHz. 
 
4. Conclusions 
Machining with ns multipulse bursts of same burst 
energy EB the ablation depth is increased by a factor of 10. 
The most dominant effect is the distribution of the energy 
on several pulses which reduces the absorption of laser 
radiation in the plasma. This enlarges the absorbed energy 
into the surface and therefore the expel of melt gets greater. 
Further the increasing pulse duration of the pulses in the 
burst lead to more effectively melt removal. The third 
reason for the total increase of the ablation depth with 
multipulses is the short interpulse separation Δt that leads 
to a pre-conditioned interaction volume that supports melt 
ejection. 
B
For the ablation with ps double pulses three sections of 
ablation are defined. For the ablation with very short 
interpulse separation 0 ns < Δt < 40 ns a deviation of the 
ablation depth and roughness Ra has been detected that has 
to be investigated in future. For interpulse separation 
between 40 ns < Δt < 400 to 700 ns the plasma interacts in 
a negative manner with the sample e.g. by increasing the 
roughness Ra. For interpulse separation Δt > 400 to 700 ns 
the ablation of double pulses equals to the ablation with 
single pulses of half burst energy EB. Therefore a cut-off 
frequency ν
B
cut-off = 1 to 3 MHz can be defined up to which 
the plasma has no detectable influence on the ablation. This 
cut-off frequency νcut-off could be a well limit for the 
maximal frequency for laser ablation with ps laser pulses. 
47 
 
JLMN-Journal of Laser Micro/Nanoengineering Vol. 2, No. 1, 2007 
 
 
Acknowledgments 
Part of this work was conducted with financial support 
of the Deutsche Forschungsgemeinschaft under contract No. 
Gi 265/4-2. 
 
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(Received: May 16, 2006, Accepted: February 2, 2007) 
 
 
48 


Investigation on Laser Micro Ablation of Steel Using Short and Ultrashort IR Multipulses

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Investigation on Laser Micro Ablation of Steel Using Short and Ultrashort IR Multipulses

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