optical components

Refractive and diffractive laser

beam shaping optics
High end components for material processing

Especially for micro machining lasers

the authors
have opened new possibilities which were
not accessible by traditional mechanical
processes (e.g. intravolume glass mark- Udo Umhofer Erwin Jäger
ing). For many applications the raw Dr. Udo Umhofer studied Dr. Erwin Jäger studied
Gaussian laser beam profiles are not opti- physics at the Technical physics and received his
mal adapted for material processing. University of Darmstadt PhD at Technical Univer-
Therefore optimized beam profiles like and completed his PhD sity of Darmstadt in the
those with homogeneous intensity (so on THz phonon spectroscopy. After a few field of Computer Generated Holograms
called Top Hat) or with ring shape are of- years as R&D manager in a gas laser com- (CGH). After two years research at the In-
ten desired to enhance process efficiency pany he co-founded and is director of Topag stitute of Technical Optics (ITO) Stuttgart,
and quality. Lasertechnik. Since 2006 he is executive he co-founded and is director of Topag La-
board member of the competence network sertechnik. Currently he is responsible for a
Recently the use of modern non-spherical for optical technologies Optence e.V. R&D project developing diffractive beam
optics, such as freeform refractive and also shaping optics for laser material processing
diffractive optics, has spread into a variety applications.
of applications. This development became
possible by new technologies allowing to Christian Bischoff
realize practically any surface shape or grat- Christian Bischoff stud-
ing structure with highest accuracy. This ied physical engineering
introduces a new degree of freedom in the at Technical University of 
design of optical systems leading to signifi- Applied Science Mittel- Topag Lasertechnik GmbH
Nieder-Ramstädter-Str. 247
cant optimization potential. One example is hessen, Friedberg with 64285 Darmstadt, Germany
the use of so called Top Hat profiles for the focus on laser physics. Phone: +49 (0)6151 425978
structuring of thin film solar cells leading to During his diploma the- Fax: +49 (0)6151 425988
E-mail: umhofer@topag.de
enhanced process speed and improved ab- sis in 2007 he concentrated on refractive and jaeger@topag.de
lation quality. Principally such profiles could diffractive optical elements for laser beam bischoff@topag.de
be generated either by using refractive or shaping at Topag Lasertechnik, where he Website: www.topag.de
diffractive beam shapers. works as project manager since 2008.

Refractive beam shapers require

ultra precision manufacturing
tiny focus spots with any desired profile but commercial use in most cases will be re-
Top Hat beam shaping optics typically trans- also a large variety of light distributions, even stricted to wafer based elements or molds
form an incoming collimated laser beam into picture generation, can be realized by such for high precision embossing in glass.
a round or square shaped uniform intensity analytical methods. An advantage of refractive beam shaping
distribution. Refractive Top Hat shapers for The more complex the required light elements is their typical high efficiency and
Gaussian (TEM00) beams are mostly freeform distribution is the more complex will be the the wide useful wavelength range. Yet there
optics and can be designed using analytical corresponding surface structure. To realize are limitations in manufacturing, as the ra-
methods based on the principle of conver- such elements ultra precision manufactur- dius of curvature at the surface cannot be
sation of energy. With this method one cal- ing (UPM) methods like diamond turning or too small. As a result not every desired light
culates for each point of the input plane a milling are necessary resulting in surface ac- distribution can be generated by refractive
local deflection angle (called “mapping”) curacy in the order of 100 nm and roughness elements. Furthermore the achievable spot
to receive the desired profile in the image better than 10 nm. UPM is rather complex sizes of refractive beam shapers are above
plane. This results in smooth and continuous and costly thus it will be usually uneconomic the diffraction limit.
surface profiles. Last but not least not only for single element production. Therefore

24 LTJ Mai 2011  Nr. 3 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 optical components

In principle any calculated refractive sur-

face or phase profile, for example an aspheric
lens, can also be realized in form of a diffrac-
tive structure. Dividing the continuous sur-
face profile of a lens into a set of surfaces with
corresponding curvatures and stepwise dis-
continuities in between will lead to a Fresnel
lens. If such curvatures are approximated
by discrete steps, typically with maximum
phase height of 2π, it will be called a diffrac-
tive structure. The simplest diffractive opti-
cal element (DOE) is a binary phase plate,
approximated by only two phase steps. It
should be emphasized that a DOE allows to
realize any phase function, also those un-
feasible for refractive optics due to the high
complexity of the required surfaces.

Advantages of diffractive beam

shapers
Traditional ray tracing methods cannot take
into account all effects introduced by dis-
cretization such as contributions from higher
diffraction orders and polarization or sam-
pling effects. In order to describe the result-
ing light distribution of diffractive optics it is
necessary to consider the wave propagation
of the transmitted complex light field. Such
“field tracing” enables to calculate inter-
ference, scattering and polarization effects
and allows the design of complex diffractive
structures. Yet there are high requirements FIGURE 1: Lithographic processing of diffractive optical elements: (a) Structure of chro-
mium (red) mask will be transferred into photoresist; (b) Structure of photoresist will
to computer memory size and CPU power
be etched into glass substrate; (c) Resulting phase profile.
for large wave fields. Although there exists
commercial “field tracing” software for dif- cal elements compared to conventional re- lar and has structure sizes in the order of one
fractive optics, nevertheless the optical sys- fractive ones. DOEs offer an additional de- to few micrometers only.
tem design is still a challenge. gree of freedom in optical system design. As Typically lithographic methods known
Besides the mentioned limitations there the optical function is realized within a thin from semiconductor manufacturing are ap-
are decisive advantages of diffractive opti- layer (micrometers or less) such elements plied. The lithographic processing consists
can be produced as thin and light weighted of the following steps (Fig. 1): Writing of
plates even with very large apertures. This chromium masks by electron beam, trans-
the company is connected with typically high laser radia- ferring the structure into photoresist and
tion resistance and enables a very compact following structuring of the element by ion
Topag Lasertechnik GmbH system design. Practically any light distribu- etching are applied to produce diffractive
Darmstadt, Germany tion can be generated. Commercial diffrac- beam shapers. With single mask processing
tive elements are presently offered e.g. as binary diffractive elements can be manu-
Topag Lasertechnik was founded in 1993 diffusers, multispot beam splitters and sam- factured. A multilevel DOE requires further
in Darmstadt, Germany and is estab- plers, multiple line or crosshair structures. processing steps. With two respectively tree
lished as major supplier for products in Functional multiplexing, for example the masks it is possible to manufacture DOE with
laser technology, laser optics and optical combination of a beam splitter and Top Hat four respectively eight phase levels yielding
measurement. The main product range shaper, in a single element is a further pos- maximum light efficiencies of approximately
covers ultrashort pulsed solid state lasers sibility of such designs yet rarely made use 80 % and 95 %. Obviously a multilevel DOE
for scientific and industrial applications. of so far. increases the complexity and expenses for
The company develops and offers re- The calculation of complex diffractive fabrication.
fractive and diffractive beam shaping structures as required for example for multi- Note that for certain applications, where
optics for laser material processing. beam splitters and beam diffusers is typically the desired light distribution is of symmet-
based on iterative methods, e.g. the Iterative ric shape, one can achieve even with binary
www.topag.de Fourier Transform Algorithm (IFTA)[1]. The DOEs light-efficient elements, otherwise
calculated surface structure is highly irregu- multistep or multilevel phase gratings are re-

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.laser-journal.de LTJ 25
 optical components

a) c) a) b)

b) d)

FIGURE 2: Schematics of a focusing system (a) without and (b) FIGURE 3: Comparison of results of CIGS solar cells scribing
with FBS-shaper, (c) the measured Top Hat profile generated using Gaussian (top) and Top Hat beam profiles (bottom): a)
by an FBS-shaper, and (d) the ablation of chrome on glass Removal of a front contact in ZnO(1μm)/CIGS/Mo/PI struc-
using an FBS-shaper with a single 6 ns pulse, ~1 µj at 1064 nm, ture; Laser PL10100/SH, 370 mW, 100 kHz, 532 nm; scanning
focal length 50 mm, input beam diameter: 3 mm. speed 4.3 m/s, single pass; b) SEM pictures of the P3 scribe in
ZnO(1μm)/CIGS/ Mo/PI structure; Laser PL10100/SH, 370 mW,
quired which are sometimes called “blazed ing Gaussian beam will 100 kHz, 532 nm; scanning speed 60 mm/s, single pass. (Credit:
Center for Physical Sciences and Technology, Vilnius, Lithuania)
gratings”. Binary phase gratings are also be scanned across and
positively used for Diffractive Variable At- focused onto the sur-
tenuators (see separate box). face of a material using either galvoscanners are widely used in low power applications,
A new type of diffractive beam shaper or rotating polygon mirrors. If the diffractive typically made from plastics by replication
designed by Topag, so called FBS- or Focus beam shaper provides a square Top Hat beam technologies. Yet polymers are not suitable
Beam Shaper [2], generates a square Top Hat profile, it shows up significant advantages for high power applications with respect to
profile for high power lasers. Main goal was for the scribing of CIGS solar cells (Fig. 3). UV-transmission, temperature stability and
to develop light efficient beam shapers with As this profile provides more homogeneous damage under intensive laser irradiation.
near diffraction limited Top Hat spot sizes ablation, borders of the ablation zone will be Therefore it is necessary to use glasses or
showing improved alignment tolerances and smoother and showing less melting effects, crystalline materials for beam shaping optics
a high depth of focus. Such requirements and the ablation does not damage underly- in laser material processing. Especially for
are essential e.g. for laser material process- ing structures. Further the scribing speed can UV-applications typically quartz glass will be
ing applications. A characteristic feature is be enhanced, as the overlap between sub- used.
that FBS shapers are single elements without sequent pulses can be set smaller than for Recent developments of material pro-
any focusing power and separated from any round Gaussian beams. cessing lasers for micro machining have
beam guiding optics. By simply introducing Diffractive elements for line and crosshair shown a strong increase in available laser
such a DOE into the existing beam path the generation as well as multispot beam splitters power. As these by far are exceeding the
initial Gaussian beam profile at focal plane
will be transformed into a diffraction limited
square shaped Top Hat profile. This makes it info
possible to switch easily between both beam
Diffractive Variable Attenuators for High Power Lasers
profiles depending on the application. The
FBS-shaper exhibits very high efficiency One example for an efficient binary ele-
(> 95 %) and homogeneity (± 2.5 %) of the ment is a Diffractive Variable Attenuator
Top Hat beam profile and also provides a (DVA) where the transmitted beam is
large focus depth similar to one of the initial partially deflected into several diffractive
Gaussian beam (Fig. 2). orders and subsequently the zero order
beam along the optical axis will be attenu-
ated. All higher orders are blocked by a
Beam shaping and beam splitting
diaphragm. Varying the diffractive struc-
for (micro) material processing
ture with rotation angle allows generating
A typical example for the advantages of Top a linear transmission function with only
Hat beam shaping optics in material process- one element. Since there is no absorption
ing is the structuring of thin films, for example DVAs can be used even with high power polarization and that no wave front aber-
during several stages (P1/P2/P3) of CIGS thin lasers. Further advantages of such attenu- rations will be introduced which is impor-
film solar cell production. In a principal setup ators are that they operate independent of tant for beam monitoring.
of a micromachining laser system the incom-

26 LTJ Mai 2011  Nr. 3 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
 optical components

FIGURE 4: Typical structures of diffractive beam splitters (left) and resulting beam pro-
files (right) are shown for (a) one grating period of a 1×9 beam splitter and (b) 4×4
grating periods of a 5×5 beam splitter.

necessary pulse energies it is recommended Above mentioned examples mainly focus

to introduce high power beam splitters to on applications requiring nearly diffraction
enable parallel processing and increase pro- limited spot sizes. For annealing, tempering
cess efficiencies. Multi beam splitters might or sensing larger homogeneous light distri-
get also key elements for ultrashort pulse la- butions in order of few millimeters are neces-
ser processing where a “cold ablation” is es- sary. Fig. 6 illustrates how a combination of
sential. The excess pulse energy might lead refractive Top Hat beam shaper with a ad-
again to melting zones (as with cw or nano- ditional spherical or cylindrical lenses leads
second pulsed lasers) which can be avoided to a homogeneously illuminated area or line
by splitting to multiple beam tracks thus pre- (similar to a Powell lens), respectively, with
serving the high quality of ultrashort pulse lateral sizes of few hundred micrometers to FIGURE 5: Functional multiplexing with
processing. The required angular splitting several centimeters. Besides other applica- diffractive elements: 5×5 beam splitting
between beams is typically in the order of tions such homogeneous line profiles are re- in combination with Donut-mode genera-
0.1 to few degrees. Diffractive beam splitters quired for particle counting or triangulation. tion (top) and 1×3 splitting in combina-
are a good choice, since they can be flexibly tion with a Top Hat profile (bottom).
realized in 1D and 2D configuration (Fig. 4).
Conclusion
As noted above the IFTA design methods Entwicklung wissenschaftlich-ökonomischer
are common to calculate such kind of beam New manufacturing methods for freeform re- Exzellenz, HA-Project-No.: 165/08-27).
shaping optics. fractive optics as well as development of new
References
It is obvious that combination of beam design techniques for diffractive elements
shaping and beam splitting, called func- have lead to a modern class of high end optics [1] O. Pipoll et al.: “Review of iterative Fourier-
tional multiplexing, is an interesting feature and make new applications accessible. Espe- transform algorithms for beam shaping ap-
for material processing. Using adapted beam cially in micro machining new beam shaping plications”, Optical Engineering 43, 2549
profiles, like Top Hat or Donut-mode, quality elements allow to enhance quality of results (2004)
of results could be improved, simultaneously and throughput, leading to reductions of [2] G. Raciukaitis et al.: “Laser Processing by
parallel processing allows higher throughput. losses and a higher precision of processing. Using Diffractive Optical Laser Beam Shap-
Multiplexing can be reached either by refrac- The FBS beam shaping optic has been de- ing Technique”, Journal of Laser Micro/
tive, diffractive or hybrid optical designs. In veloped in the frame of an R&D project sup- Nanoengineering 6, 37 (2011)
Fig. 5 diffractive multiplexing is shown for ported by Federal Government of Hessen,
applications in efficient structuring of thin (Germany) and Hessen Agentur, financed
films and multi parallel hole-drilling. with LOEWE funds (Landes-Offensive zur

FIGURE 6: A Top Hat beam shaper in combination with an additional spherical or cylindrical lens leads to a homogeneously illumi-
nated area or a homogeneous line profile. The Top Hat sizes could be scaled from few hundred micrometers to several centimeters.

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.laser-journal.de LTJ 27