Proceedings of the Fifth International WLT-Conference on Lasers in Manufacturing 2009
Munich, June 2009
Machining of Transparent Materials with Short Pulse
and Ultrashort Pulse Laser Sources
Udo Loeschner, Stefan Mauersberger, Joerg Schille, Robby Ebert, Horst Exner
Hochschule Mittweida – University of Applied Sciences, Mittweida, Germany
Abstract
This paper discusses the machining of transparent materials using two short pulse (nanoseconds)
and one ultrashort pulse (femtoseconds) laser sources. The investigations were carried out with
short pulse Nd:YAG lasers (1064 nm, 532 nm, and 355 nm) as well as a high repetition rate fslaser (1030 nm). In our experiments the laser beam was focused onto the sample with both a stationary objective and a laser scanner with an f-theta-objective.
In our study we investigated in detail the dependencies of controlled defect generation inside bulk
glass material on important process parameters like wavelength, pulse width, pulse repetition rate,
and irradiation regime. Due to a smart arrangement of these defects in lines, planes, and shells cut
surfaces can be generated. Finally, cutting of 3d parts, consisting of bulk transparent material, becomes possible.
Keywords: laser machining, transparent material, glass, short pulse
1
photons and thereby they will be accelerated. By transferring their energy to electrons in the valence band via
collisions these free electrons are able to generate more
free electrons by impact ionization. The described
mechanism results in a snowballing increase of the
density of free carriers, also called avalanche ionization.
By the interaction of thermalized electrons with the
phonon system of the solid melting and boiling of the
material is initiated. If the energy input into the material
is sufficient plasma formation sets in and material damage can occur.
By focussing laser radiation inside transparent bulk
material several effects have to be considered. At first,
due to optical aberration at the interface between air and
transparent material the focal point inside the material
will be blurred. Additionally, with ultrashort pulses
chromatic aberration has to be taken into account because of the larger spectral band with of the laser radiation. Again, by irradiating transparent materials with
high intensities, several nonlinear effects have to be
considered like self focusing, the most important effect.
Self focusing occurs above a critical laser pulse power
which is formulated for a Gaussian pulse by
Introduction
Using high intensity laser pulses locally confined permanent modifications inside transparent materials, like
changes in the refractive index, are possible by nonlinear absorption mechanisms. Thus optical components
with tailored functions; like micro lenses, Fresnel
lenses, gratings, waveguides, couplers, switches, up to
complex three-dimensional integrated optical devices
can be generated inside transparent bulk materials [1-5].
There are four processes involved in the interaction of
laser radiation with a solid: photon-electron-interaction,
electron-electron-interaction, electron-phonon-interaction, and phonon-phonon-interaction. First the electromagnetic field transfers its optical energy to electrons
during several femtoseconds. Electron-electron interaction takes place on a femtosecond to picosecond time
scale, electron-phonon interaction ranges from picoseconds to nanoseconds depending on the atomic bonds.
The phonon system relaxation takes nanoseconds up to
microseconds [6-8].
Most of the glass materials show high transparency for
laser wavelengths down to 355 nm. So no single photon
absorption process can be initiated because the photon
energy is less than the band gap of more than 4 eV for
most glass materials. However, if the material is exposed to highly intensive laser radiation, nonlinear absorption mechanisms for the generation of free electrons
become more probable. There are two mechanisms
involved: tunnel ionization due to the high field strength
and multi-photon ionization as a result of multi-photon
absorption. These electrons can absorb energy of other
Pcr = 1.8962 ⋅
λ2
4⋅π ⋅n0 ⋅n2
n (I ) = n 0 + n 2 ⋅ I (1)
where λ is the laser wavelength and n0 and n2 are the
respective linear and nonlinear indices of refraction of
the propagation medium [9,10].
1
and an off-axis camera for exact positioning of the sample. The entire focussing unit can be moved in the zdirection allowing the precise adjustment of the focal
plane on the sample surface or inside the material. In the
experiments both, an f-theta-objective with a 56 mm
focal length and a stationary focussing aspheric lens
with a 15 mm focal length, were used. The sample can
be fixed on a high precision xy-positioning stage and
thus exactly positioned and moved relative to the incident laser beam.
The optical properties of the materials used in the study
are presented in Tab. 2. BK9 is a boron-crown glass,
free of streaks, bubbles, and stress, mainly used for laser
engraving. The other glasses, BK7, B270, and Lithosil
(fused silica), are commercial optical materials produced by Schott AG.
In this paper detailed results of controlled defect generation inside bulk glass material in dependence on relevant process parameters like wavelength, pulse length,
pulse repetition rate, and irradiation regime are discussed and evaluated. Finally these results are utilized
to arrange defects in order to generate a cut surface that
results in the excision of a defined 3D part from the
bulk volume.
2
Experimental Details
In the investigations four laser systems differing in their
laser pulse length and wavelength were used: two short
pulse Nd:YVO4 lasers (1064 nm, 532 nm) from Edgewave, a DPSS UV laser (355 nm) from Coherent, and
an ultrashort pulse laser from Clark MXR Inc. Laser
parameters of all types are listed in detail in Tab. 1.
Tab. 1: Laser parameters
ns-laser
fs-laser
wavelength
1064 nm
532 nm
355 nm
1030 nm
(central)
photon
energy
1.16 eV
2.32 eV
3.49 eV
1.2 eV
min. pulse
length
6 ns
4 ns
40 ns
250 fs
(sech²)
max. repetition rate
30 kHz
30 kHz
300 kHz 25 MHz
max. pulse
energy
2.2 mJ
0.8 mJ
0.29 mJ
8 µJ
0.2 MW
7 kW
28 MW
pulse peak
0.3 MW
power
band gap
[eV]
refractive
index n0
BK7
Lithosil
B270
~4.7
4.7
9
4.5
1.5
1.51
1.45
1.51
?
3.45
3
3.4
?
3.3
4.0
3.3
Results and Discussion
3.1
Short-pulse ns-laser
As already discussed above, in transparent materials
multi-photon processes are necessary to absorb energy.
That means by irradiation with a wavelength of
1064 nm at least a four-photon excitation is necessary.
The probability of multi-photon absorption strongly
depends on the intensity of the laser radiation – twophoton absorption sets in at several MW/cm², three- and
more-photon absorption requires intensities in the
GW/cm² and TW/cm² range. A comparison of the maximum laser peak power of the nanosecond laser sources
and the critical power for self focussing (Tab. 1 vs.
Tab. 2) reveals, that the self focussing effect is not
existent.
By focussing short-pulse ns-laser radiation with the
fundamental wavelength (1064 nm) in combination with
the f-theta objective inside a block of BK 9 glass defect
formation could be obtained at pulse energies around
1 mJ corresponding to an applied intensity of the laser
radiation of 50 GW/cm². This agrees with the required
intensity range for four-photon absorption. As can be
seen in Fig.1 the produced defects consist each of a set
of radiant cracks with a dark centre representing the
original position of the laser focus within the material.
In [19] the occurring laser-material interaction is described more detailed.
Tab. 2: Optical properties of the investigated materials
[9,11-18]
BK9
3
(1060 nm)
nonl. refract.
index n2
[10-16 cm² W-1]
critical power
[MW]
200 µm
(1060 nm)
200 µm
Fig. 1: optical microscope image of defects in BK9
(1064 nm, pulse energy 1.2 mJ, pulse length 6 ns, pulse
distance 150 µm) - left: lateral view, right: axial view
The optical system consists of separated attenuators for
each laser, turning mirrors, and a focussing unit containing a laser scanner, a mount for stationary objectives,
2
wavelength for defect generation in Lithosil considerable more pulse energy is needed - at a level of 0.66 mJ
a 100% yield was achieved. The extension of the defects
is increased by 80% compared to the other glasses.
Latest experiments have been conducted by using the
third harmonic wavelength (355 nm) and the f-theta
objective. In BK9 defects with a yield of 100% can be
generated with a pulse energy of 0.04 mJ corresponding
to an intensity of 3.6 GW/cm². In theory, with the third
harmonic wavelength, analogue to the second harmonic
wavelength, a two-photon excitation is required. So the
observed drastically lower intensity value for defect
generation at the third harmonic wavelength is not explainable. If the single photon absorption curve of BK
glasses vs. the wavelength is considered, an exponential
rise of the absorption coefficient can be noticed for
wavelengths lower than 375 nm because of Urbachs
rule, which describes optical absorption in disordered
solids like disordered semiconductors and glasses
[20,21]. The coefficient is increased by a factor of 20
compared to the second harmonic wavelength as well as
the fundamental wavelength. So single photon absorption is contributing to the energy input, which results in
a lesser demand of intensity in respect to multi-photon
processes.
The defect dimensions could be reduced further down to
35 µm in lateral direction shown in Fig. 2. In addition,
the described sets of radiant cracks show a preferred
direction. Unexpectedly, the defects in axial direction
are more extended in comparison to the second harmonic wavelength. These peculiarities may be caused
by the astigmatism of the laser beam profile in the focal
region, which was attested by beam profile measurements. There exist two focal planes in axial different
positions - each focus is characterized by an elliptical
beam profile. In the experiments the preferred direction
of the radiant cracks and the orientation of the ellipse
agree very well. Also, the astigmatism is probably responsible for the increased axial elongation compared to
the second harmonic wavelength.
The minimal lateral extension of the cracks from a single defect achieved in BK 9 glass with optimized parameters is around 130 µm (Fig.1). The shape of the
defects along the axis of the incident laser beam (axial
direction) is more channel-like with an approximate
length of 400 µm. Within the same focal plane the axial
position of these “channels” varies up to 200 µm. At
this pulse energy level defect generation is not very
reproducible because of the stochastic nature of multiphoton absorption. To achieve a 100% yield of defects
the pulse energy was increased to 1.6 mJ corresponding
to an intensity value of 80 GW/cm². Simultaneously the
size of the defect dimensions increases slightly.
Investigations in BK7 glass lead to similar results in
respect of defect dimensions. However, only at a pulse
energy level of 2 mJ defects can be generated reproducibly. Results in B270 glass are equivalent to those of
BK7 glass. By contrast, in Lithosil considerably more
pulse energy above a level of 1.2 mJ is needed to generate defects. Even at the highest available pulse energy of
2.2 mJ the yield of defects is around 90%. By application of higher energy an increase up to 100% should be
achievable. The higher pulse energy level in Lithosil
can be discussed as follows: the band gap of Lithosil is
twice as large as the gap of other materials, so instead of
a 4-photon process an 8-photon excitation process is
necessary. Assuming equivalent intensity values of the
laser radiation the probability of an 8-photon-process is
considerably lower. As observed in the experiments,
with rising pulse energy, that means at higher intensities, the probability of an 8-photon process and therefore the yield of defects increases.
In experiments with the aspheric lens a 100% yield of
defects is achieved already at a pulse energy of 0.2 mJ,
which is equivalent to an intensity value of 71 GW/cm².
A comparison of the results obtained with the two focussing optics in respect of pulse energy and intensity
shows, that the intensity level is nearly the same, but the
pulse energy is reduced by a factor of eight. So the intensity level is the dominant parameter for defect generation. In addition, due to the stronger focussing using
the aspheric lens the defect dimensions can be reduced
considerably down to 75 µm in lateral direction and
100 µm in axial direction.
Analogue to the investigations discussed above experiments using the second harmonic wavelength (532 nm)
and the f-theta objective were performed. In BK9 glass
defects with a yield of 100% can be generated with a
pulse energy of 0.26 mJ corresponding to an intensity
value of 26 GW/cm². In comparison to the results obtained with the fundamental wavelength and the same
focussing optic the intensity level for defect generation
is reduced by a factor of three with the second harmonic
wavelength. Therefore only a two-photon absorption
process is necessary, by which a comparable yield is
obtained at lower intensities. The dimensions of the
defects can be reduced drastically down to 60 µm in
lateral direction and 100 µm in axial direction compared
to the fundamental wavelength. Also, the axial position
of the defects varies only in a range of 20 µm. In the
other glasses BK7 and B270 similar defect dimensions
were gained. As already observed with the fundamental
50 µm
200 µm
Fig. 2: optical microscope image of defects in BK9 (355
nm, pulse energy 40 µJ, pulse length 60 ns, pulse distance 50 µm) - left: lateral view, right: axial view
It should be noted, that the pulse energy of the laser
source can not be varied without influencing the pulse
length. The shortest pulse length of less than 40 ns requires the maximum pulse energy of the laser. At the
chosen pulse energy of 0.04 mJ the pulse length
amounts already to 60 ns. However, variation of the
3
lateral dimension [µm]
pulse energy at the shortest pulse length is necessary for
the investigations. So further efforts, focused on the
elimination of the discussed drawbacks of the laser
system, are required in order to improve the defect generation process and downsize the defect dimensions.
Additional experiments in BK7 glass reveal the same
defect dimensions in respect to BK9. Again, the pulse
energy level for a 100% yield of defects is identical.
Also, as already observed at the other wavelengths, at
the third harmonic wavelength considerably more pulse
energy is necessary for defect generation in Lithosil.
Because of the larger band gap at least a three-photon
excitation is required. Even at the highest available
pulse energy of 0.29 mJ the yield of defects is around
30%. Defect dimensions are in the same range as with
BK7 and BK9 glass. But at the higher pulse energies
required for a yield of 100%, the dimensions will probably be slightly increased.
A comparison of the respective defect dimensions obtained in the nanosecond regime with the three wavelengths is given in Fig. 3. It can be seen clearly, that the
lateral as well as the axial dimensions are strongly reduced with shorter wavelengths. At 355 nm the dimensions in lateral direction could be reduced by a factor
of 4 - the axial extension amounts to one-fifth at 532 nm
and 355 nm in comparison to 1064 nm. So the smallest
defects can be achieved with the third harmonic wavelength because of the shorter wavelength and therefore
the smaller focal spot.
axial dimension [µm]
Ultrashort pulse fs-laser radiation
As reported in [19] throughout the whole range of adjustable laser parameters (pulse length 250 fs - 3 ps,
pulse energy up to 8 μJ corresponding to 13 TW/cm²,
pulse repetition rate 200 kHz - 1.8 MHz) no single defects inside the transparent material can be generated.
During irradiation a shining channel along the laser
beam path could be observed in the focal region inside
the material. The shining channel is attributed to filament formation due to nonlinear effects like self focussing causing laser induced structural modifications
inside the transparent material [1,9]. Neither successful
have been additional tests with a CPA laser system,
delivering high pulse energies up to 0.8 mJ at low repetition rates and comparable pulse lengths between 180 fs
and 3 ps. With increasing pulse energy only strong filament formation occurs.
However, at certain parameters, that means with strongly overlapping laser pulses (<1 µm), longer pulse length
(>700 fs), and a minimum repetition rate of 500 kHz
imbricative material damage occurs – the defect formation is entirely different from the results obtained with
the nanosecond regime. As can be seen clearly in Fig. 4,
the lateral material damage extends across the whole
scanned area of 500 µm x 500 µm. Because there are no
single defects an estimation of the lateral defect dimension is not possible – in axial direction the defect dimension ranges from 80 µm to 100 µm. If the laser
pulse overlap is to strong the defect generation becomes
uncontrollable and the defects extend beyond the irradiated area.
150
BK9
BK7
100
Lithosil
50
0
1064
a)
b)
3.2
532
355
100 µm
wavelength [nm]
Fig. 4: optical microscope image of defects in BK9
(pulse energy 7.6 µJ, pulse length 2 ps, pulse distance 0.5 µm) - left: lateral view, right: axial view
600
500
BK9
400
BK7
300
Lithosil
The formation of material damage may be caused by
thermal accumulation effects. Due to strongly overlapping laser pulses, that means very short pulse distances,
successive pulses impact at virtually the same position.
Additionally, the time interval of 2 µs between two
pulses (for a pulse repetition rate of 500 kHz) and the
time for heat diffusion for glasses [7,22-24] are on the
same time scale, so heating of the material is beginning,
because the material is not able to cool down completely between two successive laser pulses. Again, the
need of longer pulses for defect generation can be discussed as follows: the excitation processes described in
the introduction chapter appear on different time scales.
First, absorption of laser radiation, i.e. generation of free
200
100
0
1064
532
100 µm
355
wavelength [nm]
Fig. 3: comparison of defect dimensions achieved with
nanosecond laser sources in different glasses using the
f-theta objective: a) lateral, b) axial
4
electrons, via multi-photon processes takes place during
the first several femtoseconds. Free electrons can absorb
single photons and thereby they will be accelerated. By
transferring their energy to valence electrons via collisions more free electrons can be generated. This process
results in a snowballing increase of free carriers, which
is called avalanche ionisation. This mechanism is starting with a time delay in respect to the multi-photon
process and takes femtoseconds up to picoseconds.
Further on the energy is transferred to the phonon system and heating is initiated – the supposed effect responsible for the observed material damage. If the laser
pulse length is to short, the excitation stops and the
avalanche process will not be activated completely. So
no material damage can occur.
3.3
3D parts
1 mm
Using these results in order to produce first 3D parts a
suitable irradiating strategy was developed. Starting at
the bottom of the part a layer by layer irradiation regime
was realized leading to a cut surface consisting of defects which encloses the part to be excised. To release
the 3D part easily from the glass bulk volume additional
well-arranged cut outs are inserted.
Fig. 6: sphere in BK9 glass (parameters: wavelength
1064 nm, pulse energy 0.25 mJ, pulse length 6 ns,
aspheric lens, lateral pulse distance 100 µm, axial
pulse distance 100 µm
1 mm
1 mm
a)
b)
200 µm
Fig. 7: cuboid in BK9 glass (parameters: wavelength
1030 nm, pulse energy 3 µJ, pulse length 700 fs, f-theta
objective, lateral pulse distance 0.5 µm, axial pulse
distance 150 µm): a) optical microscope image of the
cuboid, b) SEM micrograph of one corner
Fig. 5: pyramid in BK9 glass (parameters: wavelength
1064 nm, pulse energy 0.23 mJ, pulse length 6 ns,
aspheric lens, lateral pulse distance 75 µm, axial pulse
distance 150 µm
5
Figs. 5-7 demonstrate the current status of the developed technique. The nanosecond laser sources as well as
the femtosecond laser source are suitable tools to cut
transparent material. To characterize the surface quality
of the walls roughness measurements were conducted
with a confocal point sensor μScan from NanoFokus.
The measurements reveal, that the average roughness
values achieved with the nanosecond laser sources at the
fundamental wavelength as well as at the second harmonic wavelength amount to approximately 3 µm. By
comparing the quality of the parts produced with the
femtosecond laser source with the optimum results
obtained with short nanosecond pulses it can be observed that the surface roughness of this specimen is
three times higher. So at the present time the nanosecond laser sources are the preferred tool for cutting of
transparent material.
4
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Conclusions
This paper discusses detailed results of controlled defect
generation inside bulk glass material in dependence on
relevant process parameters. With nanosecond laser
sources above a wavelength dependent pulse energy
level defects can be generated reproducibly in various
glasses. The defects consist each of a set of radiant
cracks with a dark centre representing the original position of the laser focus within the material. The spatial
extension of the defects strongly depends on the chosen
laser wavelength. With the third harmonic wavelength
minimum dimensions in lateral direction of 35 µm were
achieved.
With the femtosecond laser source no single defect formation inside the transparent material can be
generated throughout the whole range of adjustable
laser parameters – pulse length, pulse energy, and pulse
repetition rate. Only strong formation of filaments occurs. But with strongly overlapping laser pulses
(<1 µm), longer pulse length (>700 fs), and a minimum
repetition rate of 500 kHz imbricative material damage
takes place as a result of thermal accumulation effects.
Finally, 3d parts, cut from bulk material, are presented. Roughness measurements reveal, that the specimens machined with nanosecond laser sources are
3 times smoother than the ones produced with the femtosecond laser source.
Acknowledgement
The authors gratefully acknowledge financial support of
the present work by the Bundesministerium für Bildung
und Forschung (project number 03IP506). Our thanks
go to Prof. Frank Mueller at Hochschule Mittweida
(FH) dep. MB/FWT for expert SEM recordings.
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7