-Laser Microfabrication of Diffractive Phase Elements

M. Pfeifer, F. Jahn, A. Kratsch, B. Steiger and S. Weissmantel

University of Applied Sciences Mittweida, Technikumplatz 17, 09648 Mittweida, Germany

Keywords: DPE, Diffractive Phase Element, DOE, Diffractive Optical Element, Fluorine Laser, Mask Projection.

Abstract: Fluorine laser microfabrication enables direct structuring of diffractive phase elements in fused silica. These

elements are used as beam shapers for the wide wavelength range from 248 to 1100 nm. We present

selected phase elements for laser beam shaping in the visible wavelength range. Furthermore we show the

laser beam shaping of a fiber laser with a wavelength of λ = 1070 nm. The main advantage of this

application is the much more compact design compared with the current used method of beam shaping by

conventional optics. Fluorine laser microstructuring provides an effective alternative to lithography

techniques. It is a fast and individual method to fabricate customized or prototype diffractive phase elements

in a cost-effective way.

1 INTRODUCTION

Beam shaping by diffractive phase elements (DPEs)

is currently one of the most important fields of

research on optics with almost the greatest potential

in application and innovation. In the future it will be

possible to shape any beam profile into almost every

desired beam distribution. In this way DPEs enable

the homogenization of laser beams. The main

advantage of this application is the much more

compact design compared with the current used

method of beam homogenization by conventional

optics. Furthermore, it is conceivable to design

special DPEs shaping the raw laser beam directly

into the desired beam geometry for the working

plane. In this way, masks are no longer needed. Thus

the laser power can be used more efficiently by

using the full beam cross-section.

The manufacturing of DPEs is mainly done by

photolithography. This technique is very complex,

because it has to be done in several photolithography

steps. Also it requires a various number of masks

due to the desired number of steps of the DPE. Laser

microstructuring using a mask projection technique

opens up a new way. It is a fast and individual

technique to fabricate customized or prototype

diffractive phase elements in a cost-effective way.

This is possible because, for instance, compared

with photolithography there are no special masks

necessary, which have to be manufactured for each

new DPE. Also there is only one mask needed,

because of the pixelwise structuring process. The

microstructuring is done by pulsed laser ablation

using a fluorine laser. Because of its short

wavelength in the VUV range, it enables

micromachining of wide band gap materials such as

fused silica. Furthermore, calcium fluoride is used as

material for beam shapers for the excimer laser

wavelength 157 nm and 193 nm. However, this

material is very difficult to structure by lithography

techniques because of its material properties.

Fluorine laser microstructuring provides an effective

alternative at this point.

2 F

-LASER STATION

The microstructuring is done by a fully automated

high-precision laser micromachining station. It was

developed and built by 3D-Micromac AG in

cooperation with the University of Applied Sciences

Mittweida. A fluorine laser LPF 220i of Coherent

(Deutschland) GmbH is integrated in this station.

The laser beam has a wavelength of 157 nm that

corresponds to a photon energy of 7.9 eV. The

maximum repetition rate is 200 Hz and the pulse

duration is 25 ns. The pulse energy can be varied

from 10 to 30 mJ. Because of the wavelength in the

VUV range, the beam guidance must take place in

an oxygen-free atmosphere. The special advantage

of the used laser station is a system of two

independent vacuum chambers. After evacuation

Pfeifer M., Jahn F., Kratsch A., Steiger B. and Weissmantel S..

F2-Laser Microfabrication of Diffractive Phase Elements.

DOI: 10.5220/0004713600910096

In Proceedings of 2nd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2014), pages 91-96

ISBN: 978-989-758-008-6

 2014 SCITEPRESS (Science and Technology Publications, Lda.)

these chambers are flooded with pure nitrogen (5.0)

to normal pressure.

The laser microstructuring is done by mask

projection technique. Here the mask geometry is

optically imaged onto the substrate surface by a

transmission objective with a demagnification ratio

of 26 : 1. The maximum structure size on the

substrate surface is about 225 x 225 µm² with a

resolution of < 1 µm. The maximum possible laser

pulse fluence on the substrate surface is about

7 J/cm².

3 DESIGN OF DIFFRACTIVE

PHASE ELEMENTS

Diffractive phase elements are a special kind of

diffractive optical elements. The active principle is

the modulation of the phasing of the electromagnetic

wave. The phase displacement between the points on

the wave front is provided by different optical path

lengths, which is achieved by structuring of different

step heights. This leads to the intended diffraction

effects and therefore to the generation of defined

diffraction images. Fused silica is used as substrate

material because of its good transmission properties

over a wide spectral range.

The calculation of the DPEs is based on a variant

of the well know POCS-method (projection onto

convex sets) (Ersoy, 2007). Here the given field

strength distributions are addressed as boundary

conditions at the DPE and pictorial space. Both, the

input-sided intensity distribution and the distribution

in the target plane, i.e. the desired diffraction image,

must be given. The transition between the spaces is

mathematically done by propagation operators. The

most frequently used operator is the Fourier

transformation and the inverse Fourier

transformation, respectively. Therefore, this

procedure is state of the art in various versions as so-

called iterative Fourier transform algorithm (IFTA)

(Fienup, 1982; Ripoll, Kettunen and Herzig, 2004;

Kim, Yang and Lee, 2004). The convergence of this

algorithm thereby depends on various parameters. A

good result for the phase distribution of the intended

DPE is reached after approximately 30 iteration

steps.

A detail of such a phase distribution is shown in

Figure 1. Here a defined phasing is assigned to each

gray value. The phase distribution of the DPE is

completely irregularly based on the unsymmetrical

intensity distribution in the target plane.

Figure 1: Detail of the phase distribution and target

structure of a nine level DPE producing the logo of the

University of Applied Sciences Mittweida (array of

128 x 128 pixels).

The original quasi-continuously calculated

distribution is discretized during the algorithm both

in its allocation and regarding to the possible

phasing. This creates the shown pixel-like structure.

This discretization is due to the properties of the

manufacturing technology. The higher diffraction

orders indicated in Figure 2 are a disadvantage of

this simplification. This leads to a loss of power in

laser beam shaping.

The applying of the Fourier transformation as

propagation operator between DPE and pictorial

space creates so-called Fourier elements. Their

diffraction image is basically formed in the far field

in infinite distance. However, a distance which is

very large towards the pixel size becomes practically

sufficient. In the academic literature different criteria

are defined for the quantitative determination of the

minimum distance (Lipson, Lipson and Tannhauser,

1997; Träger, 2007). Such DPEs are used for

alignment applications because of their almost

constant image quality over long distances.

With special regard to the practical application of

laser beam shaping for laser material processing

often there is defined a clear target plane, in which

the desired image should be formed sharply. This is

done by additional phase terms, which allow a

further manipulation of the diffraction image. For

instance, a spherical lens term enables the sharply

image formation in a defined target plane.

Figure 2 shows the simulated diffraction image

of a DPE producing the logo of the University of

Applied Sciences Mittweida in different projection

distances. The phase distribution of this DPE

contains a spherical lens term with a focal length of

300 mm. As a result the diffraction image is formed

PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology

sharply only in the focus. The absolute size of the

created structure depends on the projection distance.

Figure 2: Simulated diffraction image of a nine level DPE

producing the logo of the University of Applied Sciences

Mittweida (array of 256 x 256 pixels, pixel size:

7.8 x 7.8 µm

, design wavelength: 532 nm) with a

spherical lens term with a focal length of 300 mm in a

projection distance of (a) 0.1 m, (b) 0.3 m, and (c) 1 m.

The simulation of the shown diffraction images

is done by numerical simulation software based on

the real expected DPE structures. Therefor beside

VirtualLab

5.3 of LightTrans GmbH also self-

created program scripts for MATLAB

MathWorks are used. Here the advantage is, beside

the possibility of control, that the influence of

different parameters on the formed diffraction image

of the DPE can be examined. In this way the cost-

intensive manufacturing of many different DPEs by

the trial-and-error method is no longer necessary.

Beside the elementary parameters pixel size,

number of pixels, and number of steps, a major

impact on the final diffraction image is particularly

caused by manufacturing-related deviations from the

ideal calculated phase distribution. Fluctuations of

the laser fluence during the structuring process result

in deviations in the structure depth. Form deviations

of the pixels are due to defects in the imaging

system. Based on these deviations the quality of the

diffraction image declines, which is shown by

Olbrich, Fischer and Steiger (2012). An essential

characteristic of that is the formation of an intensive

peak in the middle of the diffraction orders.

Figure 3: Simulated diffraction image of a DPE having

regard to real manufacturing faults: (a) fluctuations of the

laser fluence, (b) bars between the pixels, and (c) rounded

pixel corners.

Figure 3 shows the influences of the deviations

on the known diffraction image of the university

logo. Not only incorrect structure depth (see

Figure 3(a)), but also bars between the pixels (see

Figure 3(b)) and rounded corners of the normally

quadratic pixels (see Figure 3(c)) alter the diffraction

image. In these figures the formation of the centered

peak is clearly visible in each diffraction order.

Furthermore, Olbrich, Fischer and Steiger (2012)

show that surface roughness and process-related

debris on the DPE surface lead to an increased

background noise in the surrounding of the zeroth

diffraction order. Practically a mixture of the shown

phenomena can be expected according to the

influence of each error at the manufacturing process.

Such an intensity peak is absolutely

counterproductive for beam shaping for laser

material processing. Thus strategies have to be

developed to completely prevent these peaks or at

least to reduce their negative effects. A possible

approach could be the method of CO

laser

smoothing mentioned below. Another possibility

consists in the use of the already mentioned

additional phase terms. The separation of the target

structure from the centered peak is possible by using

a linear wedge term, which causes a shift of the

target structure by a defined angle.

The theoretical phase modulations must be

transferred into an appropriate form to manufacture

the designed DPEs by fluorine laser

microstructuring. Therefor, the determined phase

information is transferred to be structured depth for

each pixel according to the design wavelength and

the refractive index of the substrate. The fluorine

laser microstructuring of the DPEs is done with

masks out of tantalum foil with quadratic aperture.

Here the mask size is selected according to its image

size in the working plane, which corresponds to the

desired pixel size.

4 RESULTS AND DISCUSSION

4.1 DPEs with a Lens Term for

Λ = 532 Nm

The DPEs are structured in Corning 7980 (fused

silica). They have nine levels with a structure height

of 120 nm per step, designed for a wavelength of

λ = 532 nm. Figure 4 shows an optical micrograph

of a DPE producing the logo of the University of

Applied Sciences Mittweida.

Figure 5 shows the diffraction image of this DPE

in a projection distance of 1.68 m.

The target structure is clearly visible, but the

diffraction orders are overlaid by a centered peak.

F2-LaserMicrofabricationofDiffractivePhaseElements

Figure 4: Optical micrograph of a nine level DPE

producing the logo of the University of Applied Sciences

Mittweida (array of 256 x 256 pixels, pixel size:

7.8 x 7.8 µm

, design wavelength: 532 nm).

Figure 5: Diffraction image of the DPE shown in Figure 4

in a projection distance of 1.68 m.

The main cause is an overlaid diffraction grating

based on undesirable bars between the pixels.

Basically, their formation can be averted. The

formation of the higher diffraction orders is due to

the discretization of the DPEs in the pixel-like

structure, as mentioned before. This was already

shown in the simulation. The formation of these

diffraction orders can only be avoided by a

continuous transition of the height profile.

The DPE shown in Figure 4 is calculated with an

image formation in the infinite. The function of a

DPE with lens term is shown in Figure 6. This DPE

differs to the previous by an additional spherical lens

term with a focal length of 300 mm (cf. Figure 2).

The diffraction image is shown in different

projection distances.

The real diffraction images are similar to the

simulated ones. It becomes clear, that just like in the

simulation the diffraction image becomes blurred

faster towards a shorter projection distance than

towards a longer one.

A comparison of the diffraction images in

Figure 5(b) and Figure 6(b) shows, that the

Figure 6: Diffraction image of the DPE described in

Figure 2 in a projection distance of (a) 0.2 m, (b) 0.3 m,

and (c) 1 m.

diffraction image of the DPE with lens term is more

detailed and has sharper contours than the DPE

without a lens term. Thus the partially blur of the

diffraction image of the DPE without a lens term is

not caused by a worse structuring. The diffraction

image should be captured in a longer projection

distance.

4.2 DPE with a Wedge Term for

Λ = 633 Nm

To separate the target structure from the centered

peak, a DPE with a linear wedge term was

structured. It is an eleven level DPE with a structure

height of 120 nm per step, designed for a

wavelength of λ = 633 nm. The target structure of

this DPE is the same as before. It is also designed

with an additional spherical lens term, but with a

focal length of 320 mm.

Figure 7: Diffraction image of an eleven level DPE

producing the logo of the University of Applied Sciences

Mittweida (array of 256 x 256 pixels, pixel size:

10 x 10 µm

, design wavelength: 633 nm) with a spherical

lens term with a focal length of 320 mm and a linear

wedge term captured in focus.

The diffraction image of the DPE is shown in

Figure 7. The target structure is shifted by a defined

angle and therefore it is completely separated from

the centered peak. The red stripe on the left side of

the image is not a part of the diffraction image. It is

a saturation effect of the CCD sensor. Also the

circular halo effect is a result of undesired

reflections at the beam expander.

PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology

4.3 Beam Shaper for a Fiber Laser

with Λ = 1070 Nm

Based on the investigations above a DPE was

manufactured to shape a fiber laser beam with a

wavelength of λ = 1070 nm into the known target

structure. The DPE has 19 levels with a structure

height of 120 nm per step. The DPE is designed with

an additional spherical lens term with a focal length

of 100 mm and a linear wedge term.

Figure 8: Diffraction image of a 19 level DPE producing

the logo of the University of Applied Sciences Mittweida

(array of 160 x 160 pixels, pixel size: 22.3 x 22.3 µm

design wavelength: 1070 nm) with a spherical lens term

with a focal length of 100 mm and a linear wedge term

captured in focus.

The diffraction image of the DPE is shown in

Figure 8. The centered peaks are not very

pronounced due to a reducing of the undesirable bars

between the pixels. There is only some background

noise in the surrounding of the zeroth diffraction

order, as mentioned before. Thus the laser beam

shaping for an ytterbium-doped fiber laser was

successfully done.

Besides a DPE for laser beam shaping of a diode

laser with a wavelength of λ = 940 nm was

manufactured. It was already tested at a laser power

up to 1 kW in cw mode for a short time without any

damage of the DPE.

4.4 CO

Laser Smoothing of DPEs

Bars could occur between the pixels, as mentioned

in chapter 4.1. These undesirable bars have a

negative impact on the diffraction image so far as

they produce a diffraction image of the raw laser

beam. This second diffraction image overlays the

desired ones (cf. Figure 5(a)). Another problem is a

roughening of the surface caused by the laser

microstructuring process. This leads to a blur of the

diffraction image. Both the reducing of the surface

roughness and the removal of the undesirable bars is

done by temperature controlled CO

laser

smoothing.

Figure 9: Three-dimensional confocal micrograph of a

DPE producing an “F” (array of 50 x 50 pixels, pixel size:

21 x 21 µm

, design wavelength: 532 nm) (a) before and

(b) after CO

laser smoothing.

Figure 9 shows the confocal micrograph of the

DPE before and after CO

laser smoothing. The

related profiles are shown in Figure 10.

Figure 10: Profile of selected pixels of the DPE shown in

Figure 9 (a) before and (b) after CO

laser smoothing.

The success of the technique is clearly visible.

The undesirable bars between the pixels are almost

completely removed. The quadratic surface

roughness could be improved from S

= 44.9 nm to

= 20.3 nm. These are averaged values over 25

pixels with different structure depths. The roughness

depends on the structure depth. The roughness of the

unstructured surface is S

= 8.9 nm.

The diffraction image of the DPE before and

after CO

laser smoothing is shown in Figure 11.

A comparison of Figure 11(a) and Figure 11(c)

shows, that the centered peaks are only weakly

visible due to the removing of the undesirable bars

between the pixels. Also the higher diffraction

orders are less pronounced after smoothing, which

comes along with an increase of the laser beam

intensity in the zeroth diffraction order. As this

diffraction order is more pronounced and has sharper

F2-LaserMicrofabricationofDiffractivePhaseElements

Figure 11: Diffraction image of the DPE shown in

Figure 9 in a projection distance of 1 m (a), (b) before and

(c), (d) after CO

laser smoothing.

contours, which can be seen in Figure 11(d)

compared with Figure 11(b). This effect is due to a

redistribution of the glass by the CO

laser

smoothing. Thus the edges of the pixels are rounded

and the pixel-like structure becomes more a kind of

continuous transition of the height profile.

5 CONCLUSIONS

The microstructuring of diffractive phase elements

by fluorine laser is possible. Both the function of

DPEs with image formation in the infinite and in a

defined projection distance could be shown.

Furthermore, the function of an additional wedge

term was shown, which separate the target structure

from the centered peak. On basis of these

investigations a DPE was manufactured to shape a

fiber laser beam, which could successfully be

shown.

The image quality of the diffraction images of

the DPEs could be improved by CO

laser

smoothing. The undesirable bars were almost

completely removed and the surface roughness

smoothed.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial

support of the present work by the Federal Ministry

of Education and Research (FKZ: 16V0053), the

European Union and the Free State of Saxony.

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Kim, H., Yang, B. and Lee, B., 2004. Iterative Fourier

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Lipson, S., Lipson, H. and Tannhauser, D., 1997. Optik,

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PHOTOPTICS2014-InternationalConferenceonPhotonics,OpticsandLaserTechnology